DOC: Fix documentation build command in contribution guide.
State: closed

- [X] closes #25478
- [X] tests added / passed
- [X] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [X] whatsnew entry


Labels:
name: Docs

Comments:
lgtm. ping on green
# [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/25479?src=pr&el=h1) Report
> Merging [#25479](https://codecov.io/gh/pandas-dev/pandas/pull/25479?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/50c40ff1afa4a4a6772225e02c320294c422ed1a?src=pr&el=desc) will **increase** coverage by `<.01%`.
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/25479/graphs/tree.svg?width=650&token=eZ4WkYLtcO&height=150&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/25479?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #25479      +/-   ##
==========================================
+ Coverage   91.75%   91.75%   +<.01%     
==========================================
  Files         173      173              
  Lines       52942    52942              
==========================================
+ Hits        48575    48576       +1     
+ Misses       4367     4366       -1
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `90.31% <ø> (ø)` | :arrow_up: |
| #single | `41.71% <ø> (-0.01%)` | :arrow_down: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/25479?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/util/testing.py](https://codecov.io/gh/pandas-dev/pandas/pull/25479/diff?src=pr&el=tree#diff-cGFuZGFzL3V0aWwvdGVzdGluZy5weQ==) | `87.66% <0%> (+0.09%)` | :arrow_up: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/25479?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/25479?src=pr&el=footer). Last update [50c40ff...890d66a](https://codecov.io/gh/pandas-dev/pandas/pull/25479?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/25479?src=pr&el=h1) Report
> Merging [#25479](https://codecov.io/gh/pandas-dev/pandas/pull/25479?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/50c40ff1afa4a4a6772225e02c320294c422ed1a?src=pr&el=desc) will **increase** coverage by `<.01%`.
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/25479/graphs/tree.svg?width=650&token=eZ4WkYLtcO&height=150&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/25479?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #25479      +/-   ##
==========================================
+ Coverage   91.75%   91.75%   +<.01%     
==========================================
  Files         173      173              
  Lines       52942    52942              
==========================================
+ Hits        48575    48576       +1     
+ Misses       4367     4366       -1
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `90.31% <ø> (ø)` | :arrow_up: |
| #single | `41.71% <ø> (-0.01%)` | :arrow_down: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/25479?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/util/testing.py](https://codecov.io/gh/pandas-dev/pandas/pull/25479/diff?src=pr&el=tree#diff-cGFuZGFzL3V0aWwvdGVzdGluZy5weQ==) | `87.66% <0%> (+0.09%)` | :arrow_up: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/25479?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/25479?src=pr&el=footer). Last update [50c40ff...890d66a](https://codecov.io/gh/pandas-dev/pandas/pull/25479?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).

@jreback 
thanks