Update transfer-manager.md

[KyronSr]

Return type of completionFuture seems to have changed since this was written.

Issue #, if available:

Simply changed to CompletedFileDownload instead of CompletedDownload

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[tkhill-AWS]

Thank you for your pull request.

I actually need to route this into the documentation via xml, so I'm going to close this pull request and create a ticket on my end.

After I saw your suggestion, I tried running the code and ran into more issues. The methods should be S3TransferManager.uploadFile()/downloadFile since those are the methods that support source/destination methods.