Update sam.md (huggingface#31130)

`mask` variable is not defined. probably a writing mistake. it should be `segmentation_map`. `segmentation_map` should be a `1` channel image rather than `RGB`.
[on a different note, the `mask_url` is the same as `raw_image`. could provide a better example.

docs/source/en/model_doc/sam.md

@@ -81,10 +81,10 @@ processor = SamProcessor.from_pretrained("facebook/sam-vit-huge")
 img_url = "https://huggingface.co/ybelkada/segment-anything/resolve/main/assets/car.png"
 raw_image = Image.open(requests.get(img_url, stream=True).raw).convert("RGB")
 mask_url = "https://huggingface.co/ybelkada/segment-anything/resolve/main/assets/car.png"
-segmentation_map = Image.open(requests.get(mask_url, stream=True).raw).convert("RGB")
+segmentation_map = Image.open(requests.get(mask_url, stream=True).raw).convert("1")
 input_points = [[[450, 600]]]  # 2D location of a window in the image
 
-inputs = processor(raw_image, input_points=input_points, segmentation_maps=mask, return_tensors="pt").to(device)
+inputs = processor(raw_image, input_points=input_points, segmentation_maps=segmentation_map, return_tensors="pt").to(device)
 with torch.no_grad():
     outputs = model(**inputs)