object API: correct "buf" v.s. "map" mismatch in *.c and *.h

object API: correct "buf" v.s. "map" mismatch in *.c and *.h

Change the name of the second argument to check_object_signature() to
be "buf" in object-file.c, making it consistent with the prototype in
cache.h

This fixes an inconsistency that's been with us since 2ade9340262 (Add
"check_sha1_signature()" helper function, 2005-04-08), and makes a
subsequent commit's diff smaller, as we'll move these API docs to
cache.h.

While we're at it fix a small grammar error in the documentation,
dropping an "an" before "in-core object-data".

Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>