Documentation/SubmittingPatches: unify whitespace/tabs for the DCO

The Developers Certificate of Origin has a mixture of tabs and white
spaces which is annoying to view if your editor explicitly views white
space characters.

Also remove any trailing white spaces found in the file.
Signed-off-by: NStefan Beller <sbeller@google.com>
Signed-off-by: NJonathan Corbet <corbet@lwn.net>