Fix up grammar

synapse/storage/events.py

@@ -556,9 +556,9 @@ class EventsStore(EventsWorkerStore):
 
             state_groups_map[ctx.state_group] = ctx.current_state_ids
 
-        # We need to map the event_ids to their state groups. First, lets
+        # We need to map the event_ids to their state groups. First, let's
-        # check if the event is one we're persisting and then we can pull the
+        # check if the event is one we're persisting, in which case we can
-        # state group from its context.
+        # pull the state group from its context.
         # Otherwise we need to pull the state group from the database.
 
         # Set of events we need to fetch groups for. (We know none of the old