Fix offset in `get_predicted_polygons_with_confidence()`

There is an issue with the offset when calling get_polygon() in get_predicted_polygons_with_confidence().

As a result, the polygon coordinates are not correct.

    for text_piece, confidence in zip(text_list, confidence_list):
        start_index += len(text_piece) + offset
        polygon, _ = get_polygon(
            text_piece,
            max_value,
-            offset,
+            start_index,
            weights,
            threshold_method=threshold_method,
            threshold_value=threshold_value,
            size=(width, height),
        )