keyval: Fix grammar comment to cover downstream prefix

According to the grammar, a key __com.redhat_foo would be parsed as
two key fragments __com and redhat_foo.  It's actually parsed as a
single fragment.  Fix the grammar.

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Message-Id: <20220218145551.892787-2-armbru@redhat.com>