Fix broken link in documentation

The link to the exception handling framework page on the System Design /
Firmware Design / Section 4.3 just links to itself, so I changed it to
link to the exception handling framework component document.
Signed-off-by: John Powell <john.powell@arm.com>
Change-Id: I6711b423a789b2b3d1921671e8497fffa8ba33d1

docs/design/firmware-design.rst

@@ -988,7 +988,7 @@ before restoring the stack and CPU state and returning from the original SMC.
 Exception Handling Framework
 ----------------------------
 
-Please refer to the `Exception Handling Framework`_ document.
+Please refer to the :ref:`Exception Handling Framework` document.
 
 Power State Coordination Interface
 ----------------------------------