grammar: it is / plural agreement

Reviewed-by: Richard Laager <rlaager@wiktel.com>
Reviewed-by: Matt Ahrens <mahrens@delphix.com>
Reviewed-by: Chris Dunlop <chris@onthe.net.au>
Signed-off-by: Josh Soref <jsoref@users.noreply.github.com>
Closes openzfs#8818

cmd/zfs/zfs_main.c

@@ -6733,8 +6733,8 @@ unshare_unmount_compare(const void *larg, const void *rarg, void *unused)
 
 /*
  * Convenience routine used by zfs_do_umount() and manual_unmount().  Given an
- * absolute path, find the entry /proc/self/mounts, verify that its a
- * ZFS filesystems, and unmount it appropriately.
+ * absolute path, find the entry /proc/self/mounts, verify that it's a
+ * ZFS filesystem, and unmount it appropriately.
  */
 static int
 unshare_unmount_path(int op, char *path, int flags, boolean_t is_manual)