To future-proof ourselves against a change in the hash, let's use the
more generic "hash mismatch" to refer to integrity problems. Note that
we do advertise this exact string in git-fsck(1). However, the message
itself is marked for translation, meaning we do not expect it to be
machine-readable.

While we're touching that documentation, let's also update it for
grammar and clarity.
Signed-off-by: NJeff King <peff@peff.net>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>