Fixes example code in docstring of pandas.DataFrame.query
- [ ] closes #21733
- [ ] tests added / passed
- [ ] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [ ] whatsnew entry
# [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/21731?src=pr&el=h1) Report
> Merging [#21731](https://codecov.io/gh/pandas-dev/pandas/pull/21731?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/10709762e95be2b98d68b95db93a287ee5af74d4?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/21731/graphs/tree.svg?height=150&src=pr&token=eZ4WkYLtcO&width=650)](https://codecov.io/gh/pandas-dev/pandas/pull/21731?src=pr&el=tree)

```diff
@@           Coverage Diff           @@
##           master   #21731   +/-   ##
=======================================
  Coverage   91.92%   91.92%           
=======================================
  Files         158      158           
  Lines       49705    49705           
=======================================
  Hits        45693    45693           
  Misses       4012     4012
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `90.3% <ø> (ø)` | :arrow_up: |
| #single | `41.99% <ø> (ø)` | :arrow_up: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/21731?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/core/frame.py](https://codecov.io/gh/pandas-dev/pandas/pull/21731/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZnJhbWUucHk=) | `97.19% <ø> (ø)` | :arrow_up: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/21731?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/21731?src=pr&el=footer). Last update [1070976...2b5a8ea](https://codecov.io/gh/pandas-dev/pandas/pull/21731?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 thanks @alphaCTzo7G 