README.md: use modules to install
README.md: use modules to install

Change-Id: I263cfd7cee957f3835841bf383d78054cc884086
Reviewed-on: https://code-review.googlesource.com/c/gocloud/+/46750
Reviewed-by: Jean de Klerk <605b93664484fed1e671a07bff68da39e73d0174@google.com>

googleapis/google-cloud-go,googleapis/google-cloud-go,googleapis/google-cloud-go,googleapis/google-cloud-go