urls.txt: avoid auto converting to hyperlink

file:///path/to/repo.git/ is converted to a hyperlink while others are
not. Put a backslash to avoid the conversion. Tested with asciidoc
8.6.5.

Signed-off-by: Nguyễn Thái Ngọc Duy <pclouds@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/urls.txt

@@ -33,7 +33,7 @@ For local repositories, also supported by Git natively, the following
 syntaxes may be used:
 
 - /path/to/repo.git/
-- file:///path/to/repo.git/
+- \file:///path/to/repo.git/
 
 ifndef::git-clone[]
 These two syntaxes are mostly equivalent, except when cloning, when