Added example of icebreakers-cleanup-crew

(I figure its low cost to just list all the possible pings, compared to the cost of people getting the command wrong or not even knowing the full set of teams possible.)

src/ice-breaker/about.md

@@ -59,6 +59,7 @@ team. For example:
 
 ```text
 @rustbot ping icebreakers-llvm
+@rustbot ping icebreakers-cleanup-crew
 ```
 
 **Note though that this should only be done by compiler team members