fix 4 lgtm alerts
Fix name of methods as `rolling_mean` instead of `rolling_median` following a43c1576ce3d94bc82f7cdd63531280ced5a9fa0.

I'm a dev on lgtm.com so I'm very biased but these were flagged [here](https://lgtm.com/projects/g/pydata/pandas/rev/a43c1576ce3d94bc82f7cdd63531280ced5a9fa0) and you can catch such issues (and more subtle ones) before they find their way into master by giving a go at [PR integration](https://lgtm.com/docs/lgtm/config/pr-integration) :)

Hope that helps! 

# [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/16881?src=pr&el=h1) Report
> Merging [#16881](https://codecov.io/gh/pandas-dev/pandas/pull/16881?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/d236f31c7cfc740549b363926580ec0c94559b25?src=pr&el=desc) will **decrease** coverage by `0.01%`.
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/16881/graphs/tree.svg?width=650&height=150&src=pr&token=eZ4WkYLtcO)](https://codecov.io/gh/pandas-dev/pandas/pull/16881?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #16881      +/-   ##
==========================================
- Coverage   90.99%   90.97%   -0.02%     
==========================================
  Files         161      161              
  Lines       49301    49301              
==========================================
- Hits        44861    44852       -9     
- Misses       4440     4449       +9
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `88.74% <ø> (ø)` | :arrow_up: |
| #single | `40.19% <ø> (-0.07%)` | :arrow_down: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/16881?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/io/gbq.py](https://codecov.io/gh/pandas-dev/pandas/pull/16881?src=pr&el=tree#diff-cGFuZGFzL2lvL2dicS5weQ==) | `25% <0%> (-58.34%)` | :arrow_down: |
| [pandas/core/frame.py](https://codecov.io/gh/pandas-dev/pandas/pull/16881?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZnJhbWUucHk=) | `97.71% <0%> (-0.1%)` | :arrow_down: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/16881?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/16881?src=pr&el=footer). Last update [d236f31...1fbd718](https://codecov.io/gh/pandas-dev/pandas/pull/16881?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/16881?src=pr&el=h1) Report
> Merging [#16881](https://codecov.io/gh/pandas-dev/pandas/pull/16881?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/d236f31c7cfc740549b363926580ec0c94559b25?src=pr&el=desc) will **decrease** coverage by `0.01%`.
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/16881/graphs/tree.svg?height=150&src=pr&token=eZ4WkYLtcO&width=650)](https://codecov.io/gh/pandas-dev/pandas/pull/16881?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #16881      +/-   ##
==========================================
- Coverage   90.99%   90.97%   -0.02%     
==========================================
  Files         161      161              
  Lines       49301    49301              
==========================================
- Hits        44861    44852       -9     
- Misses       4440     4449       +9
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `88.74% <ø> (ø)` | :arrow_up: |
| #single | `40.19% <ø> (-0.07%)` | :arrow_down: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/16881?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/io/gbq.py](https://codecov.io/gh/pandas-dev/pandas/pull/16881?src=pr&el=tree#diff-cGFuZGFzL2lvL2dicS5weQ==) | `25% <0%> (-58.34%)` | :arrow_down: |
| [pandas/core/frame.py](https://codecov.io/gh/pandas-dev/pandas/pull/16881?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZnJhbWUucHk=) | `97.71% <0%> (-0.1%)` | :arrow_down: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/16881?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/16881?src=pr&el=footer). Last update [d236f31...1fbd718](https://codecov.io/gh/pandas-dev/pandas/pull/16881?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 thanks!