What does this change do?

Clarifies the copyto command description using the wording suggested in the issue: copying a single file to a destination with a different name, for example uploading and renaming in one step.

Linked issue

Fixes #9527

For new or changed backends

Not a backend change.

Checklist

• This change is trivial OR it has been discussed and agreed in the linked issue.
• I have read the contribution guidelines.
• I have added tests for all changes in this PR if appropriate.
• I have added documentation for the changes if appropriate.
• All commit messages are in house style.
• (Backend changes only) test_all passes for this backend and I can provide a test account for the integration tester - see CONTRIBUTING.md.
• This Pull Request is ready for review.

Verification:

• gofmt -w cmd/copyto/copyto.go
• go test ./cmd/copyto -count=1
• git diff --check