Fix typo in merge doc for validate kwarg
 - [x] tests added / passed

# [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/16595?src=pr&el=h1) Report
> Merging [#16595](https://codecov.io/gh/pandas-dev/pandas/pull/16595?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/9e620bc2898d4eb1d8aa7a3a6c4f2a90d272e78e?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/16595/graphs/tree.svg?height=150&token=eZ4WkYLtcO&width=650&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/16595?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #16595      +/-   ##
==========================================
+ Coverage   90.92%   90.92%   +<.01%     
==========================================
  Files         161      161              
  Lines       49240    49240              
==========================================
+ Hits        44772    44773       +1     
+ Misses       4468     4467       -1
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `88.68% <ø> (ø)` | :arrow_up: |
| #single | `40.23% <ø> (ø)` | :arrow_up: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/16595?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/core/frame.py](https://codecov.io/gh/pandas-dev/pandas/pull/16595?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZnJhbWUucHk=) | `97.66% <ø> (ø)` | :arrow_up: |
| [pandas/core/indexes/datetimes.py](https://codecov.io/gh/pandas-dev/pandas/pull/16595?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvaW5kZXhlcy9kYXRldGltZXMucHk=) | `95.33% <0%> (+0.09%)` | :arrow_up: |

------

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> `Δ = absolute <relative> (impact)`, `ø = not affected`, `? = missing data`
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