doc: RAS: fixing broken links

There were some links in the file "ras.rst" which were broken, this
patch fixes all the broken links in this file.

Signed-off-by: Manish Pandey <manish.pandey2@arm.com>
Change-Id: I00cf080e9338af5786239a4843cb4c2e0cc9d99d
diff --git a/docs/getting_started/porting-guide.rst b/docs/getting_started/porting-guide.rst
index 45c27ad..c98f3cc 100644
--- a/docs/getting_started/porting-guide.rst
+++ b/docs/getting_started/porting-guide.rst
@@ -2729,6 +2729,8 @@
 registers x0 through x5 to do its work. The return value is 0 on successful
 completion; otherwise the return value is -1.
 
+.. _External Abort handling and RAS Support:
+
 External Abort handling and RAS Support
 ---------------------------------------