Add conditions merged to delete_head_branch

Taken from https://docs.mergify.io/examples.html

.mergify.yml

@@ -11,6 +11,7 @@ pull_request_rules:
       merge:
         method: squash
   - name: delete head branch after merge
-    conditions: []
+    conditions:
+      - merged
     actions:
       delete_head_branch: {}