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/design/firmware-design.rst b/docs/design/firmware-design.rst
index 24efabe..bddff16 100644
--- a/docs/design/firmware-design.rst
+++ b/docs/design/firmware-design.rst
@@ -2866,7 +2866,7 @@
 .. _PSCI: https://developer.arm.com/documentation/den0022/latest/
 .. _Arm ARM: https://developer.arm.com/docs/ddi0487/latest
 .. _SMC Calling Convention: https://developer.arm.com/docs/den0028/latest
-.. _Trusted Board Boot Requirements CLIENT (TBBR-CLIENT) Armv8-A (ARM DEN0006D): https://developer.arm.com/docs/den0006/latest/trusted-board-boot-requirements-client-tbbr-client-armv8-a
+.. _Trusted Board Boot Requirements CLIENT (TBBR-CLIENT) Armv8-A (ARM DEN0006D): https://developer.arm.com/docs/den0006/latest
 .. _Arm Confidential Compute Architecture (Arm CCA): https://www.arm.com/why-arm/architecture/security-features/arm-confidential-compute-architecture
 .. _AArch64 exception vector table: https://developer.arm.com/documentation/100933/0100/AArch64-exception-vector-table