doc: Update instructions to download the FVP-Base emulation package

Users should be downloading the FVP_Base_RevC-2xAEMvA_11.14_21.tgz package from
Arm's website.

Issue-Id: SCM-2575
Signed-off-by: Diego Sueiro <diego.sueiro@arm.com>
Change-Id: Ide0641106fc9bd0074c2979ab263719d85fe0172