Fix documentation code to import facebook/detr-resnet-50 model (#20491)

docs/source/en/model_doc/detr.mdx

@@ -119,7 +119,7 @@ Option 1: Instantiate DETR with pre-trained weights for entire model
 ```py
 >>> from transformers import DetrForObjectDetection
 
->>> model = DetrForObjectDetection.from_pretrained("facebook/resnet-50")
+>>> model = DetrForObjectDetection.from_pretrained("facebook/detr-resnet-50")
 ```
 
 Option 2: Instantiate DETR with randomly initialized weights for Transformer, but pre-trained weights for backbone