xen/xenbus: fix typo "notifer"

There is a spelling mistake of 'notifer' in the comment which
should be 'notifier'.

Link: https://lkml.kernel.org/r/[email protected]
Signed-off-by: WangYuli <[email protected]>
Reviewed-by: Juergen Gross <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>

Author: WangYuli

include/xen/xenbus.h

@@ -180,7 +180,7 @@ int xenbus_printf(struct xenbus_transaction t,
  * sprintf-style type string, and pointer. Returns 0 or errno.*/
 int xenbus_gather(struct xenbus_transaction t, const char *dir, ...);
 
-/* notifer routines for when the xenstore comes up */
+/* notifier routines for when the xenstore comes up */
 extern int xenstored_ready;
 int register_xenstore_notifier(struct notifier_block *nb);
 void unregister_xenstore_notifier(struct notifier_block *nb);