perf script: Fix documentation of '-f' when it should be '-F'

The documentation for perf script mixes up '-f' and '-F'. Fix it.

Signed-off-by: Adrian Hunter <adrian.hunter@intel.com>
Cc: Andi Kleen <ak@linux.intel.com>
Cc: Jiri Olsa <jolsa@redhat.com>
Link: http://lkml.kernel.org/r/None


Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>