Fix: add P2 layer 21 to yolov5-p2.yaml Detect() inputs (#4608)

Layer 21 includes the information of xsmall objects

models/hub/yolov5-p2.yaml

@@ -50,5 +50,5 @@ head:
    [[-1, 10], 1, Concat, [1]],  # cat head P5
    [-1, 3, C3, [1024, False]],  # 30 (P5/32-large)
 
-   [[24, 27, 30], 1, Detect, [nc, anchors]],  # Detect(P3, P4, P5)
+   [[21, 24, 27, 30], 1, Detect, [nc, anchors]],  # Detect(P2, P3, P4, P5)
   ]