perf script: Fix documented const'ness of perf_dlfilter_fns

perf_dlfilter_fns must not be const, because it is not.

Declaring it const can result in it being mapped read-only, causing a
segfaullt when it is written. Update documentation accordingly.

Signed-off-by: Adrian Hunter <adrian.hunter@intel.com>
Cc: Jiri Olsa <jolsa@redhat.com>
Fixes: 8defa7147d5572 ("perf script Add API for filtering via dynamically loaded shared object")
Link: https //lore.kernel.org/r/20210811101036.17986-2-adrian.hunter@intel.com
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>