[Yolov8 detector] Fix example in docstring (keras-team#2082)

* fix example in docstring

* review comments

Author: cosmo3769

keras_cv/models/object_detection/yolo_v8/yolo_v8_detector.py

@@ -353,22 +353,23 @@ class YOLOV8Detector(Task):
     ```python
     images = tf.ones(shape=(1, 512, 512, 3))
     labels = {
-        "boxes": [
+        "boxes": tf.constant([
             [
                 [0, 0, 100, 100],
                 [100, 100, 200, 200],
                 [300, 300, 100, 100],
             ]
-        ],
-        "classes": [[1, 1, 1]],
+        ], dtype=tf.float32),
+        "classes": tf.constant([[1, 1, 1]], dtype=tf.int64),
     }
+
     model = keras_cv.models.YOLOV8Detector(
         num_classes=20,
         bounding_box_format="xywh",
         backbone=keras_cv.models.YOLOV8Backbone.from_preset(
-            "yolo_v8_m_coco"
+            "yolo_v8_m_backbone_coco"
         ),
-        fpn_depth=2.
+        fpn_depth=2
     )
 
     # Evaluate model without box decoding and NMS