Minor typo (pytorch#1437)

Amended a typo in part of the description.

Co-authored-by: Brian Johnson <brianjo@fb.com>

beginner_source/data_loading_tutorial.py

@@ -186,7 +186,7 @@ def __getitem__(self, idx):
 #
 # One issue we can see from the above is that the samples are not of the
 # same size. Most neural networks expect the images of a fixed size.
-# Therefore, we will need to write some prepocessing code.
+# Therefore, we will need to write some preprocessing code.
 # Let's create three transforms:
 #
 # -  ``Rescale``: to scale the image