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 <[e-mail]>
Tested-by: Arnaldo Carvalho de Melo <[e-mail]>
Cc: Adrian Hunter <[e-mail]>
Cc: Changbin Du <[e-mail]>
Cc: Ian Rogers <[e-mail]>
Cc: Ingo Molnar <[e-mail]>
Cc: Jiri Olsa <[e-mail]>
Cc: Peter Zijlstra <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Arnaldo Carvalho de Melo <[e-mail]>

tools/perf/builtin-ftrace.c

···
 	OPT_BOOLEAN('b', "use-bpf", &ftrace.target.use_bpf,
 		    "Use BPF to measure function latency"),
 #endif
-	OPT_BOOLEAN('n', "--use-nsec", &ftrace.use_nsec,
+	OPT_BOOLEAN('n', "use-nsec", &ftrace.use_nsec,
 		    "Use nano-second histogram"),
 	OPT_PARENT(common_options),
 	};