Fix typo where we ended up with multiple `WorkerLocksHandler` (#16220)

I don't think has caused any actual issues.

Introduced in #15891

changelog.d/16220.misc

@@ -0,0 +1 @@
+Fix typo where we ended up with multiple `WorkerLocksHandler`.

synapse/server.py

@@ -914,6 +914,7 @@ class HomeServer(metaclass=abc.ABCMeta):
         """Usage metrics shared between phone home stats and the prometheus exporter."""
         return CommonUsageMetricsManager(self)
 
+    @cache_in_self
     def get_worker_locks_handler(self) -> WorkerLocksHandler:
         return WorkerLocksHandler(self)