message: Update README.md
  ss58-registry.json link is updated the previous link does not work.
diff: A list of all available Substrate networks and their applicable prefixes.
  -This list is periodically checked against the master list available at https://github.com/paritytech/substrate/blob/master/ss58-registry.json
  +This list is periodically checked against the master list available at https://github.com/paritytech/ss58-registry/blob/main/ss58-registry.json
  