docs: fix grammar typo of step 2 (#2689)

Changed "are considered" to "is considered".

src/tutorial/src/step-2/description.md

@@ -10,7 +10,7 @@ The core feature of Vue is **declarative rendering**: using a template syntax th
 
 <div class="composition-api">
 
-State that can trigger updates when changed are considered **reactive**. We can declare reactive state using Vue's `reactive()` API. Objects created from `reactive()` are JavaScript [Proxies](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Proxy) that work just like normal objects:
+State that can trigger updates when changed is considered **reactive**. We can declare reactive state using Vue's `reactive()` API. Objects created from `reactive()` are JavaScript [Proxies](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Proxy) that work just like normal objects:
 
 ```js
 import { reactive } from 'vue'