Update README.md

[stevenzamborsky]

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Fix image names in Docker run commands to include hashicorp/ prefix. Without this we see the following errors:

$ docker run \
  --network host \
  -e 'BOUNDARY_POSTGRES_URL=postgresql://postgres:[email protected]:5432/postgres?sslmode=disable' \
  boundary database init -config /boundary/config.hcl

Unable to find image 'boundary:latest' locally
docker: Error response from daemon: pull access denied for boundary, repository does not exist or may require 'docker login': denied: requested access to the resource is denied.
See 'docker run --help'.