docs(prerequisites): update mbedtls version used

With Commit@55aed7d798f3d48d6aa08d58eb46c4cda318bcfb
we have now updated to use mbedtls 3.6.0.

Update document to reflect the same.

Change-Id: I6bd8fcca795373a05bc6beb2e085d24fdd14932f
Signed-off-by: Govindraj Raja <govindraj.raja@arm.com>
1 file changed