typo in docstring (#322)

Summary:
fixed small typo in update_padded function docstring.

Pull Request resolved: https://github.com/facebookresearch/pytorch3d/pull/322

Reviewed By: gkioxari

Differential Revision: D23712324

Pulled By: nikhilaravi

fbshipit-source-id: fea3b68039644b236897c6f465cbb322c5c48085

pytorch3d/structures/meshes.py

@@ -1351,7 +1351,7 @@ class Meshes(object):
 
     def update_padded(self, new_verts_padded):
         """
-        This function allows for an pdate of verts_padded without having to
+        This function allows for an update of verts_padded without having to
         explicitly convert it to the list representation for heterogeneous batches.
         Returns a Meshes structure with updated padded tensors and copies of the
         auxiliary tensors at construction time.