perf annotate: Fix usage string

perf annotate: Fix usage string

The annotate command doesn't take non-option arguments.

In fact, it can take last argument as a symbol filter though, but that's
a special case and, IMHO, it should be discouraged in favor of the -s
option.

Cc: Ingo Molnar <mingo@elte.hu>
Cc: Paul Mackerras <paulus@samba.org>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Link: http://lkml.kernel.org/r/1325957132-10600-6-git-send-email-namhyung@gmail.com
Signed-off-by: Namhyung Kim <namhyung@gmail.com>
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>