mobile: modify instructions to pull the Docker image from the hub

Image is published at https://registry.hub.docker.com/u/golang/mobile/.

Fixes #9270

Change-Id: Ib5730a5f571bf42c53f223a6543219db3e34871d
Reviewed-on: https://go-review.googlesource.com/2158
Reviewed-by: Andrew Gerrand <adg@golang.org>