DOC: small doc fixup of missing pd.
There was a failing example again in the IPython directive.

Follow-up on https://github.com/pandas-dev/pandas/pull/24649 and https://github.com/pandas-dev/pandas/pull/24650
# [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/24659?src=pr&el=h1) Report
> Merging [#24659](https://codecov.io/gh/pandas-dev/pandas/pull/24659?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/c4994ca56b5d6af35643bd9f20d39b090acbe5ff?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/24659/graphs/tree.svg?width=650&token=eZ4WkYLtcO&height=150&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/24659?src=pr&el=tree)

```diff
@@           Coverage Diff           @@
##           master   #24659   +/-   ##
=======================================
  Coverage   92.37%   92.37%           
=======================================
  Files         166      166           
  Lines       52384    52384           
=======================================
  Hits        48391    48391           
  Misses       3993     3993
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `90.8% <ø> (ø)` | :arrow_up: |
| #single | `43.02% <ø> (ø)` | :arrow_up: |


------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/24659?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/24659?src=pr&el=footer). Last update [c4994ca...01a9d03](https://codecov.io/gh/pandas-dev/pandas/pull/24659?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/24659?src=pr&el=h1) Report
> Merging [#24659](https://codecov.io/gh/pandas-dev/pandas/pull/24659?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/c4994ca56b5d6af35643bd9f20d39b090acbe5ff?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/24659/graphs/tree.svg?width=650&token=eZ4WkYLtcO&height=150&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/24659?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #24659      +/-   ##
==========================================
- Coverage   92.37%   92.37%   -0.01%     
==========================================
  Files         166      166              
  Lines       52384    52384              
==========================================
- Hits        48391    48390       -1     
- Misses       3993     3994       +1
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `90.8% <ø> (ø)` | :arrow_up: |
| #single | `43.02% <ø> (ø)` | :arrow_up: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/24659?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/util/testing.py](https://codecov.io/gh/pandas-dev/pandas/pull/24659/diff?src=pr&el=tree#diff-cGFuZGFzL3V0aWwvdGVzdGluZy5weQ==) | `88% <0%> (-0.1%)` | :arrow_down: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/24659?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/24659?src=pr&el=footer). Last update [c4994ca...6c7ff01](https://codecov.io/gh/pandas-dev/pandas/pull/24659?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
