Fix: Replace invalid model ID with valid SegFormer checkpoint in example code

## Description

The original example code attempts to load a model ID that does not exist on the Hugging Face Hub:
```python
nvidia/segformer-b5-finetuned-ade-512-512
```
This triggers the following runtime error:
```python
OSError: nvidia/segformer-b5-finetuned-ade-512-512 is not a valid model identifier listed on https://huggingface.co/models
```

## Changes

Replaced:
```python
feature_extractor = SegformerFeatureExtractor.from_pretrained("nvidia/segformer-b5-finetuned-ade-512-512")
model = SegformerForSemanticSegmentation.from_pretrained("nvidia/segformer-b5-finetuned-ade-512-512")
```
with:
```python
feature_extractor = SegformerFeatureExtractor.from_pretrained("nvidia/segformer-b5-finetuned-ade-640-640")
model = SegformerForSemanticSegmentation.from_pretrained("nvidia/segformer-b5-finetuned-ade-640-640")
```

## Testing
The code has been successfully tested and runs without error.

## Note
This contribution is part of an ongoing research initiative to systematically identify and correct faulty example code in Hugging Face Model Cards.
We would appreciate a timely review and integration of this patch to support code reliability and enhance reproducibility for downstream users.

README.md

@@ -35,8 +35,8 @@ from transformers import SegformerFeatureExtractor, SegformerForSemanticSegmenta
 from PIL import Image
 import requests
 
-feature_extractor = SegformerFeatureExtractor.from_pretrained("nvidia/segformer-b5-finetuned-ade-512-512")
-model = SegformerForSemanticSegmentation.from_pretrained("nvidia/segformer-b5-finetuned-ade-512-512")
+feature_extractor = SegformerFeatureExtractor.from_pretrained("nvidia/segformer-b5-finetuned-ade-640-640")
+model = SegformerForSemanticSegmentation.from_pretrained("nvidia/segformer-b5-finetuned-ade-640-640")
 
 url = "http://images.cocodataset.org/val2017/000000039769.jpg"
 image = Image.open(requests.get(url, stream=True).raw)