Doc: Removed TF-M prefix from document heading

This patch removes the TF-M/Trusted Firmware prefix from the
title heading across the documentation.

Change-Id: I60e0c6e48615f67ee040a0d6dbc7d6f1873856e7
Signed-off-by: Galanakis, Minos <minos.galanakis@arm.com>
diff --git a/docs/user_guides/tfm_secure_boot.rst b/docs/user_guides/tfm_secure_boot.rst
index b29b38e..83e372a 100644
--- a/docs/user_guides/tfm_secure_boot.rst
+++ b/docs/user_guides/tfm_secure_boot.rst
@@ -1,6 +1,6 @@
-################
-TF-M secure boot
-################
+###########
+Secure boot
+###########
 For secure devices it is security critical to enforce firmware authenticity to
 protect against execution of malicious software. This is implemented by building
 a trust chain where each step in the execution chain authenticates the next