doc: fix invalid use of @itemx

make[1]: Entering directory `/home/eblake/gperf/doc'
cd . && LANG= LC_MESSAGES= LC_ALL= LANGUAGE= makeinfo --no-split gperf.texi
gperf.texi:1193: @itemx must follow @item
gperf.texi:1210: @itemx must follow @item

* doc/gperf.texi (Output Details): Use correct macro.

Signed-off-by: Eric Blake <eblake@redhat.com>