Fix markdown formatting (#18368)

Followup to #18364. The `Contributed by` should be placed in a separate
paragraph.

Author: cdce8p

CHANGELOG.md

@@ -8,6 +8,7 @@ Mypy no longer supports running with Python 3.8, which has reached end-of-life.
 When running mypy with Python 3.9+, it is still possible to type check code
 that needs to support Python 3.8 with the `--python-version 3.8` argument.
 Support for this will be dropped in the first half of 2025!
+
 Contributed by Marc Mueller (PR [17492](https://github.com/python/mypy/pull/17492)).
 
 ### Mypyc accelerated mypy wheels for aarch64