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/components/firmware-update.rst b/docs/components/firmware-update.rst
index 1ba1e1c..eda7852 100644
--- a/docs/components/firmware-update.rst
+++ b/docs/components/firmware-update.rst
@@ -494,4 +494,4 @@
 .. _Universally Unique Identifier: https://tools.ietf.org/rfc/rfc4122.txt
 .. |Flow Diagram| image:: ../resources/diagrams/fwu_flow.png
 .. |FWU state machine| image:: ../resources/diagrams/fwu_states.png
-.. _PSA FW update specification: https://developer.arm.com/documentation/den0118/a/
+.. _PSA FW update specification: https://developer.arm.com/documentation/den0118/latest/