DOC: Fix typo in timeseries.rst
Simple typo fix
# [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/15981?src=pr&el=h1) Report
> Merging [#15981](https://codecov.io/gh/pandas-dev/pandas/pull/15981?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/6c0cfff8aa643d31462de07e8a693d035fc0e77b?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/15981/graphs/tree.svg?width=650&src=pr&token=eZ4WkYLtcO&height=150)](https://codecov.io/gh/pandas-dev/pandas/pull/15981?src=pr&el=tree)

```diff
@@           Coverage Diff           @@
##           master   #15981   +/-   ##
=======================================
  Coverage      91%      91%           
=======================================
  Files         145      145           
  Lines       49581    49581           
=======================================
  Hits        45123    45123           
  Misses       4458     4458
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `88.77% <ø> (ø)` | :arrow_up: |
| #single | `40.57% <ø> (ø)` | :arrow_up: |


------

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