Fix line highlighting on web-dev page (#11251)

Be consistent with css highlight.

![image](https://github.com/user-attachments/assets/19f76503-3d66-4ddb-9803-3f92682ec3c2)

I think highlighting `shape: BoxShape.circle`, will be better?

src/content/get-started/flutter-for/web-devs.md

@@ -745,7 +745,7 @@ with [`BoxShape` enum][] for this purpose.
 }
 ```
 
-```dart highlightLines=10,13-14,18
+```dart highlightLines=11,14-15,19
 final container = Container(
   // grey box
   width: 320,