docs: use smaller image URL in vision example

Fixes #2776

The original image URL (1599px) was too large and caused a 400 error
when the API tried to download it. Changed to a smaller 640px version
which works correctly.

Author: majiayu000

README.md

@@ -77,7 +77,7 @@ With an image URL:
 
 ```python
 prompt = "What is in this image?"
-img_url = "https://upload.wikimedia.org/wikipedia/commons/thumb/d/d5/2023_06_08_Raccoon1.jpg/1599px-2023_06_08_Raccoon1.jpg"
+img_url = "https://upload.wikimedia.org/wikipedia/commons/thumb/d/d5/2023_06_08_Raccoon1.jpg/640px-2023_06_08_Raccoon1.jpg"
 
 response = client.responses.create(
     model="gpt-4o-mini",