perf ftrace latency: Remove unnecessary "--" from --use-nsec option

perf ftrace latency: Remove unnecessary "--" from --use-nsec option

The option name should not have the dashes.  Current version shows four
dashes for the option.

  $ perf ftrace latency -h

   Usage: perf ftrace [<options>] [<command>]
      or: perf ftrace [<options>] -- [<command>] [<options>]
      or: perf ftrace {trace|latency} [<options>] [<command>]
      or: perf ftrace {trace|latency} [<options>] -- [<command>] [<options>]

      -b, --use-bpf         Use BPF to measure function latency
      -n, ----use-nsec      Use nano-second histogram
      -T, --trace-funcs <func>
                            Show latency of given function

Fixes: 84005bb6148618cc ("perf ftrace latency: Add -n/--use-nsec option")
Signed-off-by: Namhyung Kim <namhyung@kernel.org>
Tested-by: Arnaldo Carvalho de Melo <acme@redhat.com>
Cc: Adrian Hunter <adrian.hunter@intel.com>
Cc: Changbin Du <changbin.du@huawei.com>
Cc: Ian Rogers <irogers@google.com>
Cc: Ingo Molnar <mingo@kernel.org>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Link: https://lore.kernel.org/r/20230525212038.3535851-1-namhyung@kernel.org
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>