doc: Updated user guide with new Mbed TLS version number
doc: Updated user guide with new Mbed TLS version number

Signed-off-by: zelalem-aweke <f8d6e612699c5d92d66f9594bab8d711d509c3f6@arm.com>
Change-Id: Ib12ecc0b283274c74cdfa57caf9e1a105dce3afe

achingupta/arm-trusted-firmware,achingupta/arm-trusted-firmware