For system prune, -f is not an alias for --filter

Signed-off-by: Misty Stanley-Jones <misty@docker.com>

docs/reference/commandline/system_prune.md

@@ -115,7 +115,7 @@ Total reclaimed space: 13.5 MB
 
 ### Filtering
 
-The filtering flag (`-f` or `--filter`) format is of "key=value". If there is more
+The filtering flag (`--filter`) format is of "key=value". If there is more
 than one filter, then pass multiple flags (e.g., `--filter "foo=bar" --filter "bif=baz"`)
 
 The currently supported filters are: