docs: fix link to TBBR specification

The former link pointed to a page which displayed the following warning
message:

  We could not find that page in the latest version, so we have taken
  you to the first page instead

Signed-off-by: Sandrine Bailleux <sandrine.bailleux@arm.com>
Change-Id: Icf9277770e38bc5e602b75052c2386301984238d
diff --git a/docs/index.rst b/docs/index.rst
index a7a5993..cdb237a 100644
--- a/docs/index.rst
+++ b/docs/index.rst
@@ -91,7 +91,7 @@
 .. _Armv7-A and Armv8-A: https://developer.arm.com/products/architecture/a-profile
 .. _Secure Monitor: http://www.arm.com/products/processors/technologies/trustzone/tee-smc.php
 .. _Power State Coordination Interface (PSCI): https://developer.arm.com/documentation/den0022/latest/
-.. _Trusted Board Boot Requirements CLIENT (TBBR-CLIENT): https://developer.arm.com/docs/den0006/latest/trusted-board-boot-requirements-client-tbbr-client-armv8-a
+.. _Trusted Board Boot Requirements CLIENT (TBBR-CLIENT): https://developer.arm.com/docs/den0006/latest
 .. _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