Fix user directory background update (#4887)

changelog.d/4887.feature

+The user directory has been rewritten to make it faster, with less chance of falling behind on a large server. 

synapse/storage/schema/delta/53/user_dir_populate.sql

@@ -23,7 +23,7 @@ INSERT INTO background_updates (update_name, progress_json, depends_on) VALUES
 
 -- Insert all users, if search_all_users is on
 INSERT INTO background_updates (update_name, progress_json, depends_on) VALUES
-    ('populate_user_directory_process_users', '{}', 'populate_user_directory_rooms');
+    ('populate_user_directory_process_users', '{}', 'populate_user_directory_process_rooms');
 
 -- Clean up staging tables
 INSERT INTO background_updates (update_name, progress_json, depends_on) VALUES