osmAccountDialog: Fix OSM signup link
As discussed in https://github.com/openstreetmap/openstreetmap-website/issues/2555, the referer parameter is being misused here, resulting in a bad redirect when the link is followed.
As discussed in https://github.com/openstreetmap/openstreetmap-website/issues/2555, the referer parameter is being misused here, resulting in a bad redirect when the link is followed.