Fix backslashes issue under Windows.
Fix backslashes issue under Windows.

This patch fixes https://github.com/StevenBlack/hosts/pull/1165#issuecomment-590111613.

This patch simply, replace the `\` Windows directory separator, with the
universal `/` of the web convention.

StevenBlack/hosts,StevenBlack/hosts,gfyoung/hosts,borisschapira/hosts,gfyoung/hosts