DOC: rm excessive backtick
- [ ] ~~closes #xxxx (Replace xxxx with the GitHub issue number)~~ only fix sphinx syntax
- [ ] [Tests added and passed](https://pandas.pydata.org/pandas-docs/dev/development/contributing_codebase.html#writing-tests) if fixing a bug or adding a new feature
- [x] All [code checks passed](https://pandas.pydata.org/pandas-docs/dev/development/contributing_codebase.html#pre-commit).
- [ ] Added [type annotations](https://pandas.pydata.org/pandas-docs/dev/development/contributing_codebase.html#type-hints) to new arguments/methods/functions.
- [ ] Added an entry in the latest `doc/source/whatsnew/vX.X.X.rst` file if fixing a bug or adding a new feature.
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I’m developing a new sphinx-lint rule to detect excessive backticks ([PR](https://github.com/sphinx-contrib/sphinx-lint/pull/139)), and it flagged some in the current document ([this GitHub search link](https://github.com/search?q=repo%3Apandas-dev%2Fpandas+%2F%5B%5E%60%5D%3A%28class%7Cmeth%29%3A%5C%60%5B%5E%5C%60%5Cs%5D*%5C%60%5C%60%2F&type=code) lists these occurences too).
This PR fixes detected cases.

Thanks @mattwang44 