Update documentation for mbed TLS v2.16

Change-Id: I1854b5830dbd48e909a4ce1b931c13fb3e997600
Signed-off-by: John Tsichritzis <john.tsichritzis@arm.com>

docs/user-guide.rst

@@ -1114,7 +1114,7 @@ images with support for these features:
    is important to use a version that is compatible with TF-A and fixes any
    known security vulnerabilities. See `mbed TLS Security Center`_ for more
    information. The latest version of TF-A is tested with tag
-   ``mbedtls-2.12.0``.
+   ``mbedtls-2.16.0``.
 
    The ``drivers/auth/mbedtls/mbedtls_*.mk`` files contain the list of mbed TLS
    source files the modules depend upon.