Add 'MessageTemplate' to possible hook_civicrm_pre objects

docs/hooks/hook_civicrm_pre.md

@@ -48,6 +48,7 @@ changing between the old and new data.
         created/edited)
     -   'Membership'
     -   'MembershipPayment'
+    -   'MessageTemplate'
     -   'Event'
     -   'Participant'
     -   'ParticipantPayment'
@@ -70,4 +71,4 @@ changing between the old and new data.
 
 -   None
 
-## Example
\ No newline at end of file
+## Example