Documentation: clean up a few misspelled word typos

Used GNU "aspell check <filename>" to review various documentation
files with the default aspell dictionary. Ignored false-positives
between american and british english.

Signed-off-by: Jacob Stopak <jacob@initialcommit.io>
Reviewed-by: Eric Sunshine <sunshine@sunshineco.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
