Remove duplicate --filter from help message for ps

The `--filter` option appears twice in the help message.
Its presence in the usage line suggests that it is valid for both ps and
service mode, which is wrong.

Signed-off-by: Harald Albers <github@albersweb.de>

compose/cli/main.py

@@ -619,7 +619,7 @@ class TopLevelCommand(object):
         """
         List containers.
 
-        Usage: ps [options] [--filter KEY=VAL] [SERVICE...]
+        Usage: ps [options] [SERVICE...]
 
         Options:
             -q                   Only display IDs