Fix syntax error / typo in JS code snippet (#1343)

Author: sjc5

content/en/guide/v11/upgrade-guide.md

@@ -121,7 +121,7 @@ options.vnode = (vnode) => {
         delete vnode.props.ref;
     }
 
-	if oldVNode) oldVNode(vnode);
+	if (oldVNode) oldVNode(vnode);
 }
 ```