DOC: update usage of randn in io.rst
- [x] closes #24148
- [ ] tests added / passed
- [ ] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [ ] whatsnew entry

# [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/24154?src=pr&el=h1) Report
> Merging [#24154](https://codecov.io/gh/pandas-dev/pandas/pull/24154?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/67faf6c32c52b55ed61b01d72841a703dcc8d210?src=pr&el=desc) will **decrease** coverage by `<.01%`.
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/24154/graphs/tree.svg?width=650&token=eZ4WkYLtcO&height=150&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/24154?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #24154      +/-   ##
==========================================
- Coverage    92.2%    92.2%   -0.01%     
==========================================
  Files         162      162              
  Lines       51700    51700              
==========================================
- Hits        47671    47670       -1     
- Misses       4029     4030       +1
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `90.6% <ø> (-0.01%)` | :arrow_down: |
| #single | `43.02% <ø> (-0.01%)` | :arrow_down: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/24154?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/core/internals/blocks.py](https://codecov.io/gh/pandas-dev/pandas/pull/24154/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvaW50ZXJuYWxzL2Jsb2Nrcy5weQ==) | `93.65% <0%> (-0.07%)` | :arrow_down: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/24154?src=pr&el=continue).
> **Legend** - [Click here to learn more](https://docs.codecov.io/docs/codecov-delta)
> `Δ = absolute <relative> (impact)`, `ø = not affected`, `? = missing data`
> Powered by [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/24154?src=pr&el=footer). Last update [67faf6c...cf97d7d](https://codecov.io/gh/pandas-dev/pandas/pull/24154?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 # [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/24154?src=pr&el=h1) Report
> Merging [#24154](https://codecov.io/gh/pandas-dev/pandas/pull/24154?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/defa8a8ed6f627310500388a477186bed995fef1?src=pr&el=desc) will **increase** coverage by `49.17%`.
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/24154/graphs/tree.svg?width=650&token=eZ4WkYLtcO&height=150&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/24154?src=pr&el=tree)

```diff
@@             Coverage Diff             @@
##           master   #24154       +/-   ##
===========================================
+ Coverage   43.02%    92.2%   +49.17%     
===========================================
  Files         162      162               
  Lines       51700    51700               
===========================================
+ Hits        22245    47670    +25425     
+ Misses      29455     4030    -25425
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `90.6% <ø> (?)` | |
| #single | `43.02% <ø> (ø)` | :arrow_up: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/24154?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/core/computation/pytables.py](https://codecov.io/gh/pandas-dev/pandas/pull/24154/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvY29tcHV0YXRpb24vcHl0YWJsZXMucHk=) | `92.37% <0%> (+0.3%)` | :arrow_up: |
| [pandas/io/pytables.py](https://codecov.io/gh/pandas-dev/pandas/pull/24154/diff?src=pr&el=tree#diff-cGFuZGFzL2lvL3B5dGFibGVzLnB5) | `92.3% <0%> (+0.92%)` | :arrow_up: |
| [pandas/util/\_test\_decorators.py](https://codecov.io/gh/pandas-dev/pandas/pull/24154/diff?src=pr&el=tree#diff-cGFuZGFzL3V0aWwvX3Rlc3RfZGVjb3JhdG9ycy5weQ==) | `93.24% <0%> (+4.05%)` | :arrow_up: |
| [pandas/compat/\_\_init\_\_.py](https://codecov.io/gh/pandas-dev/pandas/pull/24154/diff?src=pr&el=tree#diff-cGFuZGFzL2NvbXBhdC9fX2luaXRfXy5weQ==) | `58.3% <0%> (+8.1%)` | :arrow_up: |
| [pandas/core/config\_init.py](https://codecov.io/gh/pandas-dev/pandas/pull/24154/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvY29uZmlnX2luaXQucHk=) | `99.24% <0%> (+9.84%)` | :arrow_up: |
| [pandas/core/reshape/util.py](https://codecov.io/gh/pandas-dev/pandas/pull/24154/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvcmVzaGFwZS91dGlsLnB5) | `100% <0%> (+11.53%)` | :arrow_up: |
| [pandas/compat/numpy/\_\_init\_\_.py](https://codecov.io/gh/pandas-dev/pandas/pull/24154/diff?src=pr&el=tree#diff-cGFuZGFzL2NvbXBhdC9udW1weS9fX2luaXRfXy5weQ==) | `92.85% <0%> (+14.28%)` | :arrow_up: |
| [pandas/core/computation/common.py](https://codecov.io/gh/pandas-dev/pandas/pull/24154/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvY29tcHV0YXRpb24vY29tbW9uLnB5) | `85.71% <0%> (+14.28%)` | :arrow_up: |
| [pandas/core/api.py](https://codecov.io/gh/pandas-dev/pandas/pull/24154/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvYXBpLnB5) | `100% <0%> (+14.81%)` | :arrow_up: |
| [pandas/core/indexes/api.py](https://codecov.io/gh/pandas-dev/pandas/pull/24154/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvaW5kZXhlcy9hcGkucHk=) | `99% <0%> (+14.85%)` | :arrow_up: |
| ... and [119 more](https://codecov.io/gh/pandas-dev/pandas/pull/24154/diff?src=pr&el=tree-more) | |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/24154?src=pr&el=continue).
> **Legend** - [Click here to learn more](https://docs.codecov.io/docs/codecov-delta)
> `Δ = absolute <relative> (impact)`, `ø = not affected`, `? = missing data`
> Powered by [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/24154?src=pr&el=footer). Last update [defa8a8...6d0622c](https://codecov.io/gh/pandas-dev/pandas/pull/24154?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 @charlesdong1991 the azure CI is red, can you fix the error please

`doc/source/io.rst(4153,83): error E501: line too long (87 > 79 characters)` @charlesdong1991 not sure why you undid the commit that fixed the PEP8 error, but we need it to merge this. CI is failing because of it. ahh, @datapythonista , sorry, since in the last submit, the error was caused by `metadata` in `developer.rst`, and i didn't know why it was related to my change here, so i undid the commit to verify it... looks like it's unrelated, and i saw other PRs, and some of them have the same problem, probably something was wrong in master ( and seems master is healthy now)... so this time i added the change back to fix linting error, should pass the all tests now... sorry for this... i should have clarified in the commit, my last commit was just my experiment to find why it's failing... master was broken, and in those cases you can just wait until we fix it  then merge master and push again

You may have to merge the latest master to make the CI pass here, not sure. thanks, @datapythonista , get it! If the tests fail again, I will merge the latest master and push again. thanks @charlesdong1991 