why lRayEnd_NDC's z setted to 0 in misc05_picking_custom example

HI, thanks for the tutorial about mouse picking.
```	
glm::vec4 lRayEnd_NDC(
		((float)mouseX/(float)screenWidth  - 0.5f) * 2.0f,
		((float)mouseY/(float)screenHeight - 0.5f) * 2.0f,
		0.0,
		1.0f
	);
```
why lRayEnd_NDC's `z` coordinate is setted to 0? For my opinion, the far plane's z coordinate in `ndc` is 1
