[Unity] Fix ignore Jetbrains plugin folder (#3266)

Add missing slash to make pattern accurate, since the JetBrains folder is always relative to root assets folder.

Unity.gitignore

@@ -17,7 +17,7 @@
 # /[Aa]ssets/AssetStoreTools*
 
 # Autogenerated Jetbrains Rider plugin
-[Aa]ssets/Plugins/Editor/JetBrains*
+/[Aa]ssets/Plugins/Editor/JetBrains*
 
 # Visual Studio cache directory
 .vs/