Add missing to_json filter

roles/matrix-synapse/templates/synapse/homeserver.yaml.j2

@@ -1582,7 +1582,7 @@ spam_checker: {{ matrix_synapse_spam_checker|to_json }}
 
 # Uncomment to allow non-server-admin users to create groups on this server
 #
-enable_group_creation: {{ matrix_synapse_enable_group_creation }}
+enable_group_creation: {{ matrix_synapse_enable_group_creation|to_json }}
 
 # If enabled, non server admins can only create groups with local parts
 # starting with this prefix