perf inject: Fix missing guestmount option documentation

perf inject: Fix missing guestmount option documentation

The 'perf inject' documentation is missing the guestmount option. Add it.

Fixes: 97406a7e4fa6e5ca ("perf inject: Add support for injecting guest sideband events")
Signed-off-by: Adrian Hunter <adrian.hunter@intel.com>
Cc: Adrian Hunter <adrian.hunter@intel.com>
Cc: Ian Rogers <irogers@google.com>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: Namhyung Kim <namhyung@kernel.org>
Link: https://lore.kernel.org/r/20220811170411.84154-3-adrian.hunter@intel.com
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>