Materials should use `git+https`, not `GitHub+https`

[MarkLodato]

The spec recommends using SPDX Download Location, which is of the form:

git+https://...

Currently the code outputs GitHub+https://... because it uses the Build.Repository.Provider predefined variable verbatim.

Instead, the code should convert to either git, svn, or tfvc.

[gattjoe]

Will get this done tonight. Should be a straightforward fix.