Fix documentation for spice_main_file_copy_async()
Fix documentation for spice_main_file_copy_async()

'sources' should be a NULL-terminated array, but the parameter
documentation treats it as if it is a single file object.

flexVDI/spice-gtk,flexVDI/spice-gtk,SPICE/spice-gtk,fgouget/spice-gtk,SPICE/spice-gtk,SPICE/spice-gtk,SPICE/spice-gtk,fgouget/spice-gtk,fgouget/spice-gtk