Fix automatic generation of WMTS Tile Matrix Set Labels

[kring]

This was causing WMTS URLs to be built incorrectly, and so the overlay would fail to work.
Reported here:
https://community.cesium.com/t/ue5-3-unity2021/36516

[j9liu]

Thanks @kring ! I didn't test this -- don't think we have test data on hand for me to reproduce the error? -- but the change makes sense to me.

[kring]

Yeah I don't have good test data for this either. I tested it by putting http://localhost/{TileMatrix} in the url field and Test: in the tileMatrixSetLabelPrefix field. Requests for that URL would of course fail, and Cesium for Unity would print the failed URL. Before this fix, it would print http://localhost/, which is clearly wrong. After this fix, it would print http://localhost/Test:0, which is correct.