Fixed signup link for Portal prices
Fixed signup link for Portal prices

refs https://github.com/TryGhost/Team/issues/643
refs https://github.com/TryGhost/Admin/commit/dd75d831c0a9620c3ba19ca52596f7de4840d3ef

Last commit had incorrectly formatted the signup links for custom prices, fixed.

TryGhost/Ghost,TryGhost/Ghost,TryGhost/Ghost