perf script: Fix --list-dlfilters documentation

The option --list-dlfilters does use a string value.
Signed-off-by: Adrian Hunter <adrian.hunter@intel.com>
Cc: Jiri Olsa <jolsa@redhat.com>
Fixes: 638e2b99 ("perf script Add option to list dlfilters")
Link: https //lore.kernel.org/r/20210811101036.17986-4-adrian.hunter@intel.com
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>

tools/perf/Documentation/perf-script.txt

@@ -106,7 +106,7 @@ OPTIONS
 	Pass 'arg' as an argument to the dlfilter. --dlarg may be repeated
 	to add more arguments.
 
---list-dlfilters=::
+--list-dlfilters::
         Display a list of available dlfilters. Use with option -v (must come
         before option --list-dlfilters) to show long descriptions.