DOC: Removing tailing whitespaces in .rst files
- [ ] closes #xxxx
- [ ] tests added / passed
- [ ] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [ ] whatsnew entry

In #24216 we will start linting formatting errors in `.rst` files. Removing all the tailing whitespaces here, so we don't mix the cleaning with the code in that PR.
# [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/24281?src=pr&el=h1) Report
> Merging [#24281](https://codecov.io/gh/pandas-dev/pandas/pull/24281?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/040f06f731a09fc6e0663cada6697f6602b36f1d?src=pr&el=desc) will **not change** coverage.
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/24281/graphs/tree.svg?width=650&token=eZ4WkYLtcO&height=150&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/24281?src=pr&el=tree)

```diff
@@           Coverage Diff           @@
##           master   #24281   +/-   ##
=======================================
  Coverage   92.22%   92.22%           
=======================================
  Files         162      162           
  Lines       51828    51828           
=======================================
  Hits        47798    47798           
  Misses       4030     4030
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `90.62% <ø> (ø)` | :arrow_up: |
| #single | `43% <ø> (ø)` | :arrow_up: |


------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/24281?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/24281?src=pr&el=footer). Last update [040f06f...5d14ce6](https://codecov.io/gh/pandas-dev/pandas/pull/24281?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/24281?src=pr&el=h1) Report
> Merging [#24281](https://codecov.io/gh/pandas-dev/pandas/pull/24281?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/040f06f731a09fc6e0663cada6697f6602b36f1d?src=pr&el=desc) will **not change** coverage.
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/24281/graphs/tree.svg?width=650&token=eZ4WkYLtcO&height=150&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/24281?src=pr&el=tree)

```diff
@@           Coverage Diff           @@
##           master   #24281   +/-   ##
=======================================
  Coverage   92.22%   92.22%           
=======================================
  Files         162      162           
  Lines       51828    51828           
=======================================
  Hits        47798    47798           
  Misses       4030     4030
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `90.62% <ø> (ø)` | :arrow_up: |
| #single | `43% <ø> (ø)` | :arrow_up: |


------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/24281?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/24281?src=pr&el=footer). Last update [040f06f...5d14ce6](https://codecov.io/gh/pandas-dev/pandas/pull/24281?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 thanks!