docs: add missing heading to commit message section (#8079)

Author: mtrezza

CONTRIBUTING.md

@@ -20,6 +20,7 @@
   - [Parse Error](#parse-error)
   - [Parse Server Configuration](#parse-server-configuration)
 - [Pull Request](#pull-request)
+  - [Commit Message](#commit-message)
   - [Breaking Change](#breaking-change)
 - [Merging](#merging)
   - [Breaking Change](#breaking-change-1)
@@ -305,6 +306,8 @@ Introducing new [Parse Server configuration][config] parameters requires the fol
 
 ## Pull Request
 
+### Commit Message
+
 For release automation, the title of pull requests needs to be written in a defined syntax. We loosely follow the [Conventional Commits](https://www.conventionalcommits.org) specification, which defines this syntax:
 
 ```