Fix default guidance scale

The model page has default guidance scale to 8.0 which is incorrect. This PR fixes that to 4.0.

README.md

@@ -110,7 +110,7 @@ image = pipe(
     width=width,
     height=height,
     num_inference_steps=50,
-    guidance_scale=8.0,
+    guidance_scale=4.0,
     generator=torch.Generator(device="cuda").manual_seed(42),
 ).images[0]