Fix element name

I think it is sourcedGUID. See http://www.imsglobal.org/LTI/v1p1p1/ltiIMGv1p1p1.html#_Toc316828557.
Merge pull request #3 from michaelwheeler/patch-2

Fix element name
Merge pull request #4 from KAPx/develop

typo in field name sourceGUID to sourcedGUID