irqchip/gic-v3: Fix typos in PMR/RPR SCR_EL3.FIQ handling explanation

The GICv3 driver explanation related to PMR/RPR and SCR_EL3.FIQ
secure/non-secure priority handling contains a couple of typos.

Fix them.

Signed-off-by: Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>
Cc: Marc Zyngier <maz@kernel.org>
Signed-off-by: Marc Zyngier <maz@kernel.org>
Link: https://lore.kernel.org/r/20210121182252.29320-1-lorenzo.pieralisi@arm.com