Documentation: Add Userland/ to qtcreator include path

Author: https://github.com/debdutdeb 🔰
Commit: https://github.com/SerenityOS/serenity/commit/7527b52efc
Pull-request: https://github.com/SerenityOS/serenity/pull/12525

Documentation/UsingQtCreator.md

@@ -19,6 +19,7 @@ First, make sure you have a working toolchain and can build and run SerenityOS.
 
 ```
 .
+Userland/
 Userland/Services/
 Userland/Libraries/
 Userland/Libraries/LibC/