trace: fix prof -h

Currently it fails with:

Traceback (most rTraceback (most recent call last):
  File "scripts/trace.py", line 433, in <module>
    args = parser.parse_args()
...
  File "/usr/lib/python2.7/argparse.py", line 601, in _expand_help
    return self._get_help_string(action) % params
ValueError: incomplete format

Signed-off-by: Tomasz Grabiec <tgrabiec@cloudius-systems.com>
Signed-off-by: Pekka Enberg <penberg@cloudius-systems.com>