doc: fix repeated words

doc: fix repeated words

Inspired by 21416f0a07 ("restore: fix typo in docs", 2019-08-03), I ran
"git grep -E '(\b[a-zA-Z]+) \1\b' -- Documentation/" to find other cases
where words were duplicated, e.g. "the the", and in most cases removed
one of the repeated words.

There were many false positives by this grep command, including
deliberate repeated words like "really really" or valid uses of "that
that" which I left alone, of course.

I also did not correct any of the legitimate, accidentally repeated
words in old RelNotes.

Signed-off-by: Mark Rushakoff <mark.rushakoff@gmail.com>
Acked-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>