fix: correct typo in cameras.md (#1501)

Summary:
If my understanding is right, prp_screen[1] should be 32 rather than 48.

Pull Request resolved: https://github.com/facebookresearch/pytorch3d/pull/1501

Reviewed By: shapovalov

Differential Revision: D45044406

Pulled By: bottler

fbshipit-source-id: 7dd93312db4986f4701e642ba82d94333466b921

docs/notes/cameras.md

@@ -85,7 +85,7 @@ cameras_ndc = PerspectiveCameras(focal_length=fcl_ndc, principal_point=prp_ndc)
 # Screen space camera
 image_size = ((128, 256),)    # (h, w)
 fcl_screen = (76.8,)          # fcl_ndc * min(image_size) / 2
-prp_screen = ((115.2, 48), )  # w / 2 - px_ndc * min(image_size) / 2, h / 2 - py_ndc * min(image_size) / 2
+prp_screen = ((115.2, 32), )  # w / 2 - px_ndc * min(image_size) / 2, h / 2 - py_ndc * min(image_size) / 2
 cameras_screen = PerspectiveCameras(focal_length=fcl_screen, principal_point=prp_screen, in_ndc=False, image_size=image_size)
 ```