Docs: Rename RSS to RSE

Renames Runtime Security Subsytem (RSS) to Runtime Security Engine (RSE)
in the docs. This patch does not affect paths or code.

Signed-off-by: Jamie Fox <jamie.fox@arm.com>
Change-Id: Iecf170beef5ea694bd8eb682b877370a5eacda05
diff --git a/docs/design_docs/booting/tfm_secure_boot.rst b/docs/design_docs/booting/tfm_secure_boot.rst
index 9cb0ced..c6e8bd6 100644
--- a/docs/design_docs/booting/tfm_secure_boot.rst
+++ b/docs/design_docs/booting/tfm_secure_boot.rst
@@ -267,7 +267,7 @@
 +---------------------+-----------------+---------------+----------+----------------+--------------+
 | BL5340 DVK          | Yes             | Yes           | Yes      | No             | No           |
 +---------------------+-----------------+---------------+----------+----------------+--------------+
-| RSS                 | No              | No            | No       | No             | Yes          |
+| RSE                 | No              | No            | No       | No             | Yes          |
 +---------------------+-----------------+---------------+----------+----------------+--------------+
 
 .. [1] To disable BL2, please set the ``BL2`` cmake option to ``OFF``