Fix grammar

See #18725 (comment)

Author: MadLittleMods

scripts-dev/mypy_synapse_plugin.py

@@ -98,7 +98,7 @@ def check_prometheus_metric_instantiation(ctx: FunctionSigContext) -> CallableTy
     ensures metrics are correctly separated by homeserver.
 
     There are also some metrics that apply at the process level, such as CPU usage,
-    Python garbage collection, Twisted reactor tick time which shouldn't have the
+    Python garbage collection, and Twisted reactor tick time, which shouldn't have the
     `SERVER_NAME_LABEL`. In those cases, use use a type ignore comment to disable the
     check, e.g. `# type: ignore[missing-server-name-label]`.
     """