LG-11882: Rename Rake task for disposable email domains

[aduth]

🎫 Ticket

LG-11882

🛠 Summary of changes

Makes a few revisions to the Rake script for populating the disposable email domains table.

1. Per intent of the ticket, rename as "disposable email domains"
2. Avoid quotes in usage example which can produce an error when included
3. Reword from "URL" to "path" since the expected parameter is not a fully-qualified URL, it is expected to be a path fragment handled by Identity::Hostdata.secrets_s3.readfile

Once merged, I will plan to update the corresponding rollplan document accordingly.