docs(prerequisites): upgrade to Mbed TLS 2.28.1

In anticpation of the next Trusted Firmware release update the to newest
2.x Mbed TLS library [1].

Note that the Mbed TLS project published version 3.x some time ago.
However, as this is a major release with API breakages, upgrading to
this one might require some more involved changes in TF-A, which we are
not ready to do. We shall upgrade to Mbed TLS 3.x after the v2.8 release
of TF-A.

[1] https://github.com/Mbed-TLS/mbedtls/tree/v2.28.1

Change-Id: I7594ad062a693d2ecc3b1705e944dce2c3c43bb2
Signed-off-by: Daniel Boulby <daniel.boulby@arm.com>
diff --git a/docs/getting_started/prerequisites.rst b/docs/getting_started/prerequisites.rst
index 0ed8517..65a66b6 100644
--- a/docs/getting_started/prerequisites.rst
+++ b/docs/getting_started/prerequisites.rst
@@ -67,7 +67,7 @@
 The following libraries are required for Trusted Board Boot and Measured Boot
 support:
 
-- mbed TLS == 2.28.0 (tag: ``mbedtls-2.28.0``)
+- mbed TLS == 2.28.1 (tag: ``mbedtls-2.28.1``)
 
 These tools are optional: