doc fix load_point_cloud => load_pointcloud

Summary:
spelling errors in io.md

see https://github.com/facebookresearch/pytorch3d/discussions/1450

Reviewed By: davidsonic

Differential Revision: D43269978

fbshipit-source-id: 4bbe1f00bdeda4c51b7620e7b9cc065840303530

docs/notes/io.md

@@ -7,20 +7,20 @@ sidebar_label: File IO
 There is a flexible interface for loading and saving point clouds and meshes from different formats.
 
 The main usage is via the `pytorch3d.io.IO` object, and its methods
-`load_mesh`, `save_mesh`, `load_point_cloud` and `save_point_cloud`.
+`load_mesh`, `save_mesh`, `load_pointcloud` and `save_pointcloud`.
 
 For example, to load a mesh you might do
 ```
 from pytorch3d.io import IO
 
 device=torch.device("cuda:0")
-mesh = IO().load_mesh("mymesh.ply", device=device)
+mesh = IO().load_mesh("mymesh.obj", device=device)
 ```
 
 and to save a pointcloud you might do
 ```
 pcl = Pointclouds(...)
-IO().save_point_cloud(pcl, "output_pointcloud.obj")
+IO().save_pointcloud(pcl, "output_pointcloud.ply")
 ```
 
 For meshes, this supports OBJ, PLY and OFF files.
@@ -31,4 +31,4 @@ In addition, there is experimental support for loading meshes from
 [glTF 2 assets](https://github.com/KhronosGroup/glTF/tree/master/specification/2.0)
 stored either in a GLB container file or a glTF JSON file with embedded binary data.
 This must be enabled explicitly, as described in
-`pytorch3d/io/experimental_gltf_io.ply`.
+`pytorch3d/io/experimental_gltf_io.py`.