release notes: typo fixes

Signed-off-by: Michael J Gruber <git@drmicha.warpmail.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
Git 1.9.0

Signed-off-by: Junio C Hamano <gitster@pobox.com>
Sync with 1.9.0

* master:
  Git 1.9.0
  release notes: typo fixes