Fix link issue under Windows.

Fix link issue under Windows.

We never saw this, as we are not Windows user ourselves but there is
a real issue.

Indeed, as mentionned by @XhmikosR at
https://github.com/StevenBlack/hosts/pull/1163#issuecomment-590057635
the web links are not correctly formated. Indeed, as we use `sep` to
detect the right directory separator, it's also written into the
`readmeData.json` file.

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