config.txt: fix typo

Signed-off-by: Nicolas Dermine <nicolas.dermine@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
Documentation/config.txt: fix minor typo

Add a missing article at the beginning of a sentence, and rephrase
slightly.

Signed-off-by: Thomas Quinot <thomas@quinot.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>