"**" means bold in ASCIIDOC, so we need to escape it. This is similar
to 8447dc89 (gitignore.txt: fix documentation of "**" patterns -
2013-11-07)
Signed-off-by: NNguyễn Thái Ngọc Duy <pclouds@gmail.com>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>