docs: signal has to be called (angular#58816)

PR Close angular#58816

Author: danymarques

adev/src/content/guide/signals/linked-signal.md

@@ -45,7 +45,7 @@ const shippingOptions = signal(['Ground', 'Air', 'Sea']);
 const selectedOption = linkedSignal(() => shippingOptions()[0]);
 console.log(selectedOption()); // 'Ground'
 
-selectedOption.set(shippingOptions[2]);
+selectedOption.set(shippingOptions()[2]);
 console.log(selectedOption()); // 'Sea'
 
 shippingOptions.set(['Email', 'Will Call', 'Postal service']);