docs: Add RAS framework documentation

Change-Id: Ibf2b21b12ebc0af5815fc6643532a3be9100bf02
Signed-off-by: Jeenu Viswambharan <jeenu.viswambharan@arm.com>
diff --git a/docs/porting-guide.rst b/docs/porting-guide.rst
index bef4af6..4f1638a 100644
--- a/docs/porting-guide.rst
+++ b/docs/porting-guide.rst
@@ -2646,8 +2646,8 @@
 If you're trying to debug crashes in BL1, you can call the console_xx_core_flush
 function exported by some console drivers from here.
 
-Extternal Abort handling and RAS Support
-----------------------------------------
+External Abort handling and RAS Support
+---------------------------------------
 
 Function : plat_ea_handler
 ~~~~~~~~~~~~~~~~~~~~~~~~~~