Update the old vacumm URL on the vacumm GH page

@stefraynaud even if `vacumm` has been replaced with `xoa`, it would be nice/useful to still be able to access the old documentation from the main GH page

The old ifremer links seem broken, but I have found a working https://umr-lops-vacumm.ifremer.fr/ site

Don't know if this is the current correct link, though