Fix typo in chamfer loss docstring (#862)

Summary:
y_lengths is about `y`, not `x`.

Pull Request resolved: https://github.com/facebookresearch/pytorch3d/pull/862

Reviewed By: bottler

Differential Revision: D31304434

Pulled By: patricklabatut

fbshipit-source-id: 1db4cd57677fc018c229e02172f95ffa903d75eb

pytorch3d/loss/chamfer.py

@@ -91,7 +91,7 @@ def chamfer_distance(
         x_lengths: Optional LongTensor of shape (N,) giving the number of points in each
             cloud in x.
         y_lengths: Optional LongTensor of shape (N,) giving the number of points in each
-            cloud in x.
+            cloud in y.
         x_normals: Optional FloatTensor of shape (N, P1, D).
         y_normals: Optional FloatTensor of shape (N, P2, D).
         weights: Optional FloatTensor of shape (N,) giving weights for