credential: fix grammar

There was a lot going on behind the scenes when the vulnerability and
possible solutions were discussed. Grammar was not a primary focus,
that's why this slipped in.

Reviewed-by: Jonathan Nieder <jrnieder@gmail.com>
Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Reviewed-by: Carlo Marcelo Arenas Belón <carenas@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
