DOC: fix versionchanged blank line usage (#49117)
- [ ] closes #49117 
- [ ] [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
- [ ] 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.

@datapythonista thank you for the review. Yes, docs built locally are rendered as expected ("Changed in version 0.25.0"). However, the CI doc build failed for a reason that looks unrelated to the changes introduced by the commit (URLError: <urlopen error [Errno 104] Connection reset by peer>). Thanks! Connection errors happen sometimes, let's rerun the CI and see if we're lucky. thx @mathpn 