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>