docs: Fixes code example in "Your first scene" (#3285)

docs/getting-started/your-first-scene.mdx

@@ -59,6 +59,7 @@ We'll create a new mesh using a **BoxGeometry** and a **MeshStandardMaterial** w
     <boxGeometry />
     <meshStandardMaterial />
   </mesh>
+</Canvas>
 ```
 
 Let's pause for a moment to understand exactly what is happening here. The code we just wrote is the equivalent to this three.js code: