Fix typos in include/

Reviewed-by: Ryan Moeller <ryan@ixsystems.com>
Reviewed-by: Richard Laager <rlaager@wiktel.com>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Andrea Gelmini <andrea.gelmini@gelma.net>
Closes #9238

include/sys/zfs_acl.h

@@ -62,7 +62,7 @@ struct znode_phys;
 /*
  * All ACEs have a common hdr.  For
  * owner@, group@, and everyone@ this is all
- * thats needed.
+ * that's needed.
  */
 typedef struct zfs_ace_hdr {
 	uint16_t z_type;