Fix typo in timeseries documentation
@adamjstewart Thanks! # [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/15666?src=pr&el=h1) Report
> Merging [#15666](https://codecov.io/gh/pandas-dev/pandas/pull/15666?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/e0b37f9bb40e2d27629c573bb985d75360282cd4?src=pr&el=desc) will **decrease** coverage by `0.02%`.
> The diff coverage is `n/a`.


```diff
@@            Coverage Diff             @@
##           master   #15666      +/-   ##
==========================================
- Coverage   91.03%   91.01%   -0.03%     
==========================================
  Files         143      143              
  Lines       49382    49382              
==========================================
- Hits        44956    44946      -10     
- Misses       4426     4436      +10
```


| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/15666?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/io/gbq.py](https://codecov.io/gh/pandas-dev/pandas/compare/e0b37f9bb40e2d27629c573bb985d75360282cd4...2f9ff2a9c2a43208c88d54ffbb6f3eef85b48bfd?src=pr&el=tree#diff-cGFuZGFzL2lvL2dicS5weQ==) | `25% <0%> (-58.34%)` | :x: |
| [pandas/tools/merge.py](https://codecov.io/gh/pandas-dev/pandas/compare/e0b37f9bb40e2d27629c573bb985d75360282cd4...2f9ff2a9c2a43208c88d54ffbb6f3eef85b48bfd?src=pr&el=tree#diff-cGFuZGFzL3Rvb2xzL21lcmdlLnB5) | `93.61% <0%> (-0.34%)` | :x: |
| [pandas/core/frame.py](https://codecov.io/gh/pandas-dev/pandas/compare/e0b37f9bb40e2d27629c573bb985d75360282cd4...2f9ff2a9c2a43208c88d54ffbb6f3eef85b48bfd?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZnJhbWUucHk=) | `97.87% <0%> (-0.1%)` | :x: |
| [pandas/core/common.py](https://codecov.io/gh/pandas-dev/pandas/compare/e0b37f9bb40e2d27629c573bb985d75360282cd4...2f9ff2a9c2a43208c88d54ffbb6f3eef85b48bfd?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvY29tbW9uLnB5) | `91.3% <0%> (+0.33%)` | :white_check_mark: |

------

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> Powered by [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/15666?src=pr&el=footer). Last update [e0b37f9...2f9ff2a](https://codecov.io/gh/pandas-dev/pandas/compare/e0b37f9bb40e2d27629c573bb985d75360282cd4...2f9ff2a9c2a43208c88d54ffbb6f3eef85b48bfd?el=footer&src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).