Fix a small formatting bug

The code example used "python3" in the markdown as the language, but this prevents it from showing up with syntax highlighting. I've just changed it to "python" which fixes the error.

README.md

@@ -34,7 +34,7 @@ The original implementation had two variants: one using a ResNet image encoder a
 
 ### Use with Transformers
 
-```python3
+```python
 from PIL import Image
 import requests