Fix help message for migrate command

[k4ji]

Replaces #329 to resolve CI issues related to forked PRs.

What does this PR do?

This PR updates the help message for the migrate command to clarify that it applies to migrations between Datadog organizations, not just across datacenters.

Review checklist (to be filled by reviewers)

• Feature or bug fix MUST have appropriate tests (unit, integration, etc...)
• PR title must be written as a CHANGELOG entry (see why)
• Files changes must correspond to the primary purpose of the PR as described in the title (small unrelated changes should have their own PR)
• PR must have one changelog/ label attached. If applicable it should have the backward-incompatible label attached.
• PR should not have do-not-merge/ label attached.
• If Applicable, issue must have kind/ and severity/ labels attached at least.