Updates the Roles page title (elastic#134880)

To better differentiate between built-in roles and roles that you can create yourself, this page is renamed to Built-in roles.

Related to [elastic#2738](elastic/docs-content#2738)

Author: yetanothertw

docs/reference/elasticsearch/roles.md

@@ -5,7 +5,7 @@ applies_to:
   stack: all
 ---
 
-# Roles [built-in-roles]
+# Built-in roles [built-in-roles]
 
 This section provides detailed **reference information** for {{es}} roles.