RELEASING.md: Update doc with releasetool usage
RELEASING.md: Update doc with releasetool usage

Change-Id: I7469d3d84790c8a85a0af370d2cbe4357538c7db
Reviewed-on: https://code-review.googlesource.com/c/34574
Reviewed-by: kokoro <2ac7b1f3fa578934c95181d4272be0d3bca00121@google.com>
Reviewed-by: Jean de Klerk <605b93664484fed1e671a07bff68da39e73d0174@google.com>

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