fix code example in mgp-str doc (#22219)

Co-authored-by: yue kun <yuekun.wp@alibaba-inc.com>

docs/source/en/model_doc/mgp-str.mdx

@@ -54,7 +54,7 @@ into a single instance to both extract the input features and decode the predict
 >>> url = "https://i.postimg.cc/ZKwLg2Gw/367-14.png"
 >>> image = Image.open(requests.get(url, stream=True).raw).convert("RGB")
 
->>> pixel_values = processor(image, return_tensors="pt").pixel_values
+>>> pixel_values = processor(images=image, return_tensors="pt").pixel_values
 >>> outputs = model(pixel_values)
 
 >>> generated_text = processor.batch_decode(outputs.logits)['generated_text']