Change obsolete read_image to decode_image in data_tutorial.py (pytorch#3586)

Corrected quoted code so it reflects the code at line 137.

## Description
One word change, fixing documentation. The example previously did not
reflect the code it quoted.

cc @albanD @jbschlosser

Co-authored-by: Svetlana Karslioglu <[email protected]>

Author: 2 people

beginner_source/basics/data_tutorial.py

@@ -190,7 +190,7 @@ def __len__(self):
 
 def __getitem__(self, idx):
     img_path = os.path.join(self.img_dir, self.img_labels.iloc[idx, 0])
-    image = read_image(img_path)
+    image = decode_image(img_path)
     label = self.img_labels.iloc[idx, 1]
     if self.transform:
         image = self.transform(image)