Doc: Adjusted tfm_secure_irq_handling document headers

This patch modifies the header levels for the
tfm_secure_irq_handling document, to match the rest of the
documentation.

Change-Id: Id0b6f7156d4001941e1062fdea68e9c7f1bcf88d
Signed-off-by: Galanakis, Minos <minos.galanakis@arm.com>
1 file changed