perf stat: Fix help message for --metric-no-threshold option

perf stat: Fix help message for --metric-no-threshold option

Copy-paste error led to help message for metric-no-threshold repeating
that of metric-no-merge.

Fixes: 1fd09e299bdd434b ("perf metric: Add --metric-no-threshold option")
Reported-by: Stephane Eranian <eranian@google.com>
Signed-off-by: Ian Rogers <irogers@google.com>
Cc: Adrian Hunter <adrian.hunter@intel.com>
Cc: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Cc: Ian Rogers <irogers@google.com>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: Mark Rutland <mark.rutland@arm.com>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Link: https://lore.kernel.org/r/20231129223540.2247030-1-irogers@google.com
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>