Fix warnings in porting-guide.rst

Fix below warnings appeared in porting-guide.rst
WARNING: Title underline too short.

Change-Id: Ibc0eba0da72a53a5f9b61c49a8bf7a10b17bc3b8
Signed-off-by: Manish V Badarkhe <Manish.Badarkhe@arm.com>
1 file changed