Fix typo in comment from #11255. (#11276)

changelog.d/11276.bugfix

+Fix rolling back Synapse version when using workers.

synapse/storage/prepare_database.py

@@ -142,7 +142,7 @@ def prepare_database(
                     databases=databases,
                 )
             elif version_info.current_version < SCHEMA_VERSION:
-                # If the DB is on an older version than we expect the we refuse
+                # If the DB is on an older version than we expect then we refuse
                 # to start the worker (as the main process needs to run first to
                 # update the schema).
                 raise UpgradeDatabaseException(