Porting Guide: Remove useless escape characters

Replace all occurences of escaped underscore characters by plain ones.
This makes the text version of the porting guide easier to read and
grep into.

Change-Id: I7bf3b292b686be4c6d847a467b6708ac16544c90
Signed-off-by: Sandrine Bailleux <sandrine.bailleux@arm.com>