chore: update changelog.

Author: spencer-tb

docs/CHANGELOG.md

@@ -12,7 +12,7 @@ Test fixtures for use by clients are available for each release on the [Github r
 - Tests for the Prague fork are now marked as stable will be included in the `fixtures_stable.tar.gz` tarball from now on.
 - Tests for the Osaka fork are now included in the `fixtures_develop.tar.gz` tarball.
 
-### 💥 Important Change for `fill` Users
+#### 💥 Important Change for `fill` Users
 
 The output behavior of `fill` has changed ([#1608](https://github.com/ethereum/execution-spec-tests/pull/1608)):