Fix capitalization in the 'Fingerprint' tag for XML of serialization of SSH keys configuration

There seem to be capitalization typo for the `<Fingerprint/>` tag of the SSH keys config serialization.

Apparently there is no test for this part of the SDK so I did not update the tests.

However I am not sure whether the error is in the Python SDK or on the server side as the [documentation of the API](http://msdn.microsoft.com/en-us/library/windowsazure/jj157194.aspx) has the same typo.

Without my fix I get a BadRequest complaining about empty or null fingerprint similar to the one reported by this user:

http://social.msdn.microsoft.com/Forums/en-US/WAVirtualMachinesforWindows/thread/8cc8210c-cff2-42b8-a68c-f9df3116e7b2