Doc: Remove useless escape characters

Just like has been done in the porting guide a couple of patches
earlier, kill all escaped underscore characters in all documents.

Change-Id: I7fb5b806412849761d9221a6ce3cbd95ec43d611
Signed-off-by: Sandrine Bailleux <sandrine.bailleux@arm.com>
diff --git a/docs/platform-compatibility-policy.rst b/docs/platform-compatibility-policy.rst
index b33c4e7..b23d539 100644
--- a/docs/platform-compatibility-policy.rst
+++ b/docs/platform-compatibility-policy.rst
@@ -1,5 +1,5 @@
 TF-A Platform Compatibility Policy
-======================================================
+==================================
 
 
 .. section-numbering::