fix(docs): fix `FF-A` substitution

In this change the `FFA` substitution has been renamed to `FF-A`, as
well as the term it substitutes to - the `FFA` term does not exist.

Change-Id: I0c33d00d82a5498f7088e6a2b088a0006dfe7f65
Signed-off-by: Chris Kay <chris.kay@arm.com>
1 file changed