Fix typos in purge api & doc

* It's supposed to be purge_local_events, not ..._history
* Fix the doc to have valid json

docs/admin_api/purge_history_api.rst

@@ -23,5 +23,5 @@ To delete local events as well, set ``delete_local_events`` in the body:
 .. code:: json
 
    {
-       "delete_local_events": True,
+       "delete_local_events": true
    }

synapse/rest/client/v1/admin.py

@@ -131,9 +131,7 @@ class PurgeHistoryRestServlet(ClientV1RestServlet):
 
         body = parse_json_object_from_request(request, allow_empty_body=True)
 
-        delete_local_events = bool(
-            body.get("delete_local_history", False)
-        )
+        delete_local_events = bool(body.get("delete_local_events", False))
 
         yield self.handlers.message_handler.purge_history(
             room_id, event_id,