infra/common: refresh ACS documentation

Few updates for the ACS test procedure document
  - Shell command is corrected.
  - Modified the instructions for the second phase of the test.
  - The mount command in the “retrieve the test result” session,
    file extension is missing for luv-live-image-gpt.img. Fixed
    that.

Signed-off-by: Tony K Nadackal <tony.nadackal@arm.com>
Change-Id: I314b9549881d89259b9ade9a79b20cfa61d30d2b