docs: fix copy 'n' paste error in stop_services docstring (#1457)

This was just a copy-n-paste error, presumably from copying from the
`start_services` docstring.

Author: benhoyt

ops/pebble.py

@@ -2173,7 +2173,7 @@ def stop_services(
         timeout: float = 30.0,
         delay: float = 0.1,
     ) -> ChangeID:
-        """Stop services by name and wait (poll) for them to be started.
+        """Stop services by name and wait (poll) for them to be stopped.
 
         Args:
             services: Non-empty list of services to stop.