Minor fix in instancing-test-scene

was using methode translate for matrix4f instances in a wrong way (did
not affec results, since i was bruteforcely passing some values which
the method would handle by itself). BASICALLY: this 'translate' method
of Matrix4f expects being provided by an vector3f argument which
describes a translation in space (x,y,z) coords. There is no need for
passing a 4th argument - its rather smelling doing so, so keep this in
mind.