docs/credential: minor clarity fixups

docs/credential: minor clarity fixups

The text in git-credential(1) was copied from
technical/api-credentials, so it still talks about the
input/output format as coming from git to the helper. Since
the surrounding text already indicates that this format is
used for reading and writing with git credential, we can
just remove the extraneous confusing bits.

Signed-off-by: Jeff King <peff@peff.net>
Acked-by: Matthieu Moy <Matthieu.Moy@imag.fr>
Signed-off-by: Junio C Hamano <gitster@pobox.com>