DOC: Fix title capitalization in documentation files (#32550)
Corrected title capitalization in various .rst files to match the standard of capitalizing only the first word

- doc\source\whatsnew\v0.12.0.rst
- doc\source\whatsnew\v0.25.0.rst
- doc\source\whatsnew\v1.2.0.rst
- doc\source\whatsnew\v3.0.0.rst

Fixes part of issue https://github.com/pandas-dev/pandas/issues/32550.










Fix title capitalization in documentation files Thanks @Wong2333 