FIx the navigation title on the Elastic Agent built-in alerts page (#4159)

FIx the navigation title.
Relates to #4072

Author: alaudazzi

reference/fleet/alert-templates.md

@@ -5,7 +5,7 @@ applies_to:
 products:
   - id: fleet
   - id: elastic-agent
-navigation_title: Built-in alerts and templates
+navigation_title: Elastic Agent built-in alerts
 ---
 
 # Elastic Agent built-in alerts [built-in-alerts]