modified url for the cart pole graph

Author: gjkennedy

website/docs/tutorials/cart-pole.md

@@ -464,7 +464,7 @@ The interactive graph displays:
 
 <div style={{textAlign: 'center', margin: '1rem 0'}}>
   <iframe 
-    src={/img/cart_pole_graph.html}
+    src="/img/cart_pole_graph.html"
     width="100%" 
     height="800" 
     style={{border: 'none'}}