Update getting datacap instructions

This may be waiting on a clearer pathway in the new world of allocators, but I believe the instructions at https://docs.filecoin.io/basics/how-storage-works/filecoin-plus are currently inaccurate. (At the very least, I don't think https://filplus.fil.org/#/ gives a list of entities -- at least it has no SPs and no obvious way to find allocators), and I believe the filplus.storage path doesn't currently work, as it awaits a rewrite.