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>

docs/global_substitutions.txt

@@ -16,7 +16,7 @@
 .. |EHF| replace:: :term:`EHF`
 .. |FCONF| replace:: :term:`FCONF`
 .. |FDT| replace:: :term:`FDT`
-.. |FFA| replace:: :term:`FFA`
+.. |FF-A| replace:: :term:`FF-A`
 .. |FIP| replace:: :term:`FIP`
 .. |FVP| replace:: :term:`FVP`
 .. |FWU| replace:: :term:`FWU`