CHANGELOG.md: Fix link to an issue #238.

Author: mity

CHANGELOG.md

@@ -10,7 +10,7 @@ Fixes:
    Fix quadratic time behavior caused by one-by-one walking over block lines
    instead of calling `md_lookup_line()`.
 
- - [#237](https://github.com/mity/md4c/issues/237):
+ - [#238](https://github.com/mity/md4c/issues/238):
    Fix quadratic time and output size behavior caused by malicious misuse of
    link reference definitions.