fix class number (#4012)

paddlex/repo_apis/PaddleDetection_api/configs/PP-DocLayout_plus-L.yaml

@@ -18,7 +18,7 @@ eval_size: [800, 800]
 
 # Dataset
 metric: COCO
-num_classes: 23
+num_classes: 20
 
 worker_num: 4