Doc: Fix a formatting glitch

While converting the interrupt framework design document from Markdown
to reStructuredText format, see commit 6f625747 ("Convert
documentation to reStructuredText"), the structure of the document was
changed and one of the titles got lost. Fix that.

Change-Id: Ib3e6c1f9f01f16108fde3b89ceb624bd54049d2f
Signed-off-by: Sandrine Bailleux <sandrine.bailleux@arm.com>