index.html: target the git-snapshots releases

Previously, the Git for Windows snapshots were hosted on Azure Blobs.
Now they are hosted in GitHub releases in the
https://github.com/git-for-windows/git-snapshots repository.

This trick was performed by running the `adjust-hrefs.js` script.

Signed-off-by: Johannes Schindelin <[email protected]>

Author: dscho