Update index.md

Add load event listener to window to match the descriptive text below

files/en-us/web/api/canvas_api/tutorial/basic_usage/index.md

@@ -103,7 +103,7 @@ Here is a minimalistic template, which we'll be using as a starting point for la
           const ctx = canvas.getContext("2d");
         }
       }
-      draw();
+      window.addEventListener("load", draw);
     </script>
   </body>
 </html>