Fix changelog formatting

Author: JonRowe

Changelog.md

@@ -71,7 +71,7 @@ Enhancements:
 * Add configuration for an error exit code (to disambiguate errored builds from failed builds
   by exit status). (Dana Sherson, #2749)
 
-# 3.9.3 / 2020-09-30
+### 3.9.3 / 2020-09-30
 [Full Changelog](http://github.com/rspec/rspec-core/compare/v3.9.2...v3.9.3)
 
 Bug Fixes: