perf docs: Fix man page formatting for 'perf lock'

This makes "CONTENTION" a top level section (rather than a subsection of
"INFO").

Fixes: 79079f21 ("perf lock: Add -k and -F options to 'contention' subcommand")
Reviewed-by: James Clark <james.clark@arm.com>
Signed-off-by: Nick Forrington <nick.forrington@arm.com>
Tested-by: Ian Rogers <irogers@google.com>
Cc: Adrian Hunter <adrian.hunter@intel.com>
Cc: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: Mark Rutland <mark.rutland@arm.com>
Cc: Namhyung Kim <namhyung@kernel.org>
Link: https://lore.kernel.org/r/20231102161117.49533-1-nick.forrington@arm.comSigned-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>

tools/perf/Documentation/perf-lock.txt

@@ -119,7 +119,7 @@ INFO OPTIONS
 
 
 CONTENTION OPTIONS
---------------
+------------------
 
 -k::
 --key=<value>::