docs(prerequisites): update MbedTLS version to 3.6.1

This new update to the LTS branch of MbedTLS provides minor
enhancements and bug fixes; including some security
fixes, and a fix to a compilation warning which
previously affected TF-A.
Full patch notes to this MbedTLS update can be found at
https://github.com/Mbed-TLS/mbedtls/releases/tag/mbedtls-3.6.1.

Change-Id: I1a68dfcb52a8361c1689cb6ef12d265a6462fda3
Signed-off-by: Ryan Everett <ryan.everett@arm.com>
diff --git a/docs/getting_started/prerequisites.rst b/docs/getting_started/prerequisites.rst
index 6a0241f..42b1ec4 100644
--- a/docs/getting_started/prerequisites.rst
+++ b/docs/getting_started/prerequisites.rst
@@ -31,7 +31,7 @@
 Clang/LLVM               11.0.0
 Device Tree Compiler     1.4.7
 GNU make                 3.81
-mbed TLS\ [#f1]_         3.6.0
+mbed TLS\ [#f1]_         3.6.1
 Node.js [#f2]_           16
 OpenSSL                  1.0.0
 Poetry [#f2]_            1.3.2