Update PROMPT_GUIDE.md

docs/PROMPT_GUIDE.md

@@ -20,7 +20,7 @@ Then you can draw a box around the child's face and **emphasize that area** with
 
 ![comb](https://github.com/jhj0517/sam2-playground/blob/master/docs/prompt_with_box_and_point_combination.png)
 
-Remember that you can't use the box and point combination prompt in SAM-2. You can only use a single combination at a time.
+Remember that you can't use the multiple box and point combinations prompt in SAM-2. You can only use one combination at a time.
 
  ### Prompt with negative points
 You can also use **negative points** (red dots) to segment a more specific part. <br>