gitignore.txt: fix spelling of "backslash"

Signed-off-by: Ben North <ben@redfrontdoor.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
Merge branch 'nd/gitignore-trailing-whitespace' into jch

* nd/gitignore-trailing-whitespace:
  gitignore.txt: fix spelling of "backslash"