cmd: zfs: fix missing mention of zfs diff -h

Fixes: 344bbc82e7

Reviewed-by: Damian Szuberski <szuberskidamian@gmail.com>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Richard Yao <richard.yao@alumni.stonybrook.edu>
Signed-off-by: Ahelenia Ziemiańska <nabijaczleweli@nabijaczleweli.xyz>
Closes #14224

cmd/zfs/zfs_main.c

@@ -396,7 +396,7 @@ get_usage(zfs_help_t idx)
 	case HELP_RELEASE:
 		return (gettext("\trelease [-r] <tag> <snapshot> ...\n"));
 	case HELP_DIFF:
-		return (gettext("\tdiff [-FHt] <snapshot> "
+		return (gettext("\tdiff [-FHth] <snapshot> "
 		    "[snapshot|filesystem]\n"));
 	case HELP_BOOKMARK:
 		return (gettext("\tbookmark <snapshot|bookmark> "