[Fix doc examples] Add missing from_pretrained (#15044)

* fix doc example - ValueError: Parameter config should be an instance of class `PretrainedConfig`

* Update src/transformers/models/segformer/modeling_segformer.py

Co-authored-by: NielsRogge <48327001+NielsRogge@users.noreply.github.com>

* update

Co-authored-by: ydshieh <ydshieh@users.noreply.github.com>
Co-authored-by: NielsRogge <48327001+NielsRogge@users.noreply.github.com>

src/transformers/models/segformer/modeling_segformer.py

@@ -490,8 +490,8 @@ def forward(self, pixel_values, output_attentions=None, output_hidden_states=Non
         >>> from PIL import Image
         >>> import requests
 
-        >>> feature_extractor = SegformerFeatureExtractor.from_pretrained("nvidia/segformer-b0-finetuned-ade-512-512")
-        >>> model = SegformerModel("nvidia/segformer-b0-finetuned-ade-512-512")
+        >>> feature_extractor = SegformerFeatureExtractor.from_pretrained("nvidia/mit-b0")
+        >>> model = SegformerModel.from_pretrained("nvidia/mit-b0")
 
         >>> url = "http://images.cocodataset.org/val2017/000000039769.jpg"
         >>> image = Image.open(requests.get(url, stream=True).raw)