irq/irqdomain: Fix typo in the comment on top of __irq_domain_alloc_irqs()

The word 'number' has been misspelt in the comment on top of
_irq_domain_alloc_irqs().

Signed-off-by: Julien Grall <julien.grall@arm.com>
Signed-off-by: Marc Zyngier <marc.zyngier@arm.com>