Update README.md

Example in README documentation out of date for v.0.8.0.  __iter__() yields a length 3 tuple.  Example shows a length 2 tuple.

Change proposed: add image_name to example.

Reference: From API docs

```
__iter__() ¶
Iterate over the images and annotations in the dataset.

Yields:

Type	Description
Iterator[Tuple[str, np.ndarray, Detections]]	
Iterator[Tuple[str, np.ndarray, Detections]]: An iterator that yields tuples containing the image name, the image data, and its corresponding annotation.
```

README.md

@@ -141,7 +141,7 @@ pip install -e ".[dev]"
 - Loop over dataset entries
 
     ```python
-    >>> for image, labels in dataset:
+    >>> for name, image, labels in dataset:
     ...     print(labels.xyxy)
 
     array([[404.      , 719.      , 538.      , 884.5     ],