FIX typo in deprecation message of `deprecate_kwarg` decorator
- [x] All [code checks passed](https://pandas.pydata.org/pandas-docs/dev/development/contributing_codebase.html#pre-commit).
- [x] Added an entry in the latest `doc/source/whatsnew/v2.1.0.rst`

Previously, the error message looks has an additional quotation mark, which I assume is a **typo**:
```
FutureWarning: the 'quantile'' keyword is deprecated, use 'q' instead.
```
Now it looks like this:
```
FutureWarning: the 'quantile' keyword is deprecated, use 'q' instead.
```

Btw, does this fix need a changelog?
thx @Charlie-XIAO, no need for a changelog entry