irq: Fix comment typo ist->is

Signed-off-by: Javi Merino <javi.merino@arm.com>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>

kernel/irq/manage.c

@@ -1281,7 +1281,7 @@ EXPORT_SYMBOL(free_irq);
  *	and to set up the interrupt handler in the right order.
  *
  *	If you want to set up a threaded irq handler for your device
- *	then you need to supply @handler and @thread_fn. @handler ist
+ *	then you need to supply @handler and @thread_fn. @handler is
  *	still called in hard interrupt context and has to check
  *	whether the interrupt originates from the device. If yes it
  *	needs to disable the interrupt on the device and return