feat(fwu): update the URL links for the FWU specification

Update the links for accessing the FWU Multi Bank update specification
to point to the latest revision of the specification.

Change-Id: I25f35556a94ca81ca0a7463aebfcbc2d84595e8f
Signed-off-by: Sughosh Ganu <sughosh.ganu@linaro.org>
diff --git a/docs/index.rst b/docs/index.rst
index cdb237a..c05c0a5 100644
--- a/docs/index.rst
+++ b/docs/index.rst
@@ -95,4 +95,4 @@
 .. _System Control and Management Interface (SCMI): http://infocenter.arm.com/help/topic/com.arm.doc.den0056a/DEN0056A_System_Control_and_Management_Interface.pdf
 .. _Software Delegated Exception Interface (SDEI): http://infocenter.arm.com/help/topic/com.arm.doc.den0054a/ARM_DEN0054A_Software_Delegated_Exception_Interface.pdf
 .. _SMC Calling Convention: https://developer.arm.com/docs/den0028/latest
-.. _PSA FW update specification: https://developer.arm.com/documentation/den0118/a/
+.. _PSA FW update specification: https://developer.arm.com/documentation/den0118/latest/