docs/aemfvp-a: update the steps for installing repo

Update the repo installation steps, in addition mention the path to
find the boot logs, and fix some grammar issues in the documentations.

Signed-off-by: Selvarasu Ganesan <selvarasu.ganesan@arm.com>
Change-Id: I66711b12c3e2ab532fe89d89156405acbbbd794c