feat(docs): update Mbed-TLS version to v2.28.5

Update Mbed-TLS to the latest v2.28 LTS branch version (2.28.5).

Signed-off-by: Yann Gautier <yann.gautier@st.com>
Change-Id: Ibf27d775be2a434c0ab0d5db349a00c19e83a662
diff --git a/docs/getting_started/prerequisites.rst b/docs/getting_started/prerequisites.rst
index b3d745e..ec547b5 100644
--- a/docs/getting_started/prerequisites.rst
+++ b/docs/getting_started/prerequisites.rst
@@ -71,7 +71,7 @@
 The following libraries are required for Trusted Board Boot and Measured Boot
 support:
 
-- mbed TLS == 2.28.1 (tag: ``mbedtls-2.28.1``)
+- mbed TLS == 2.28.5 (tag: ``mbedtls-2.28.5``)
 
 These tools are optional: