of: irq: fix a trivial typo in a doc comment

Diverged from what the code does with commit 530210c7

 ("of/irq: Replace
of_irq with of_phandle_args").

Signed-off-by: Lubomir Rintel <lkundrak@v3.sk>
Signed-off-by: Rob Herring <robh@kernel.org>