Fix syntax for accessing depth in README example

Avoid AttributeError: 'dict' object has no attribute 'depth'

README.md

@@ -47,7 +47,7 @@ image = Image.open(requests.get(url, stream=True).raw)
 
 # inference
 outputs = depth_estimator(image)
-depth = outputs.depth
+depth = outputs['depth']
 ```
 For more code examples, we refer to the [documentation](https://huggingface.co/transformers/main/model_doc/zoedepth.html#).