fix(docs): replace deprecated urls under tfa/docs

Fixed internal links refrenced inside tfa/docs.
Followed https://www.sphinx-doc.org/en/master/usage/restructuredtext/roles.html#ref-role
for instrustion on how to link sections within other documents.

Signed-off-by: Thaddeus Serna <thaddeus.gonzalez-serna@arm.com>
Change-Id: I8e7c090d98951b1e3d393ab5b1d6bcdaa1865c6f
diff --git a/docs/threat_model/threat_model.rst b/docs/threat_model/threat_model.rst
index 71ec9b1..41ec57c 100644
--- a/docs/threat_model/threat_model.rst
+++ b/docs/threat_model/threat_model.rst
@@ -109,6 +109,8 @@
   +-----------------+--------------------------------------------------------+
 
 
+.. _threat_analysis:
+
 ***************
 Threat Analysis
 ***************