Fix indefinite articles usage (#1903)

* Fix grammar

* Update src/tutorial/src/step-9/description.md

Co-authored-by: Fuqiao Xue <xfq@w3.org>

Co-authored-by: Fuqiao Xue <xfq@w3.org>

Author: de-oz

src/guide/extras/reactivity-in-depth.md

@@ -361,7 +361,7 @@ The general idea of integrating Vue's reactivity system with an external state m
 
 ### Immutable Data
 
-If you are implementing a undo / redo feature, you likely want to take a snapshot of the application's state on every user edit. However, Vue's mutable reactivity system isn't best suited for this if the state tree is large, because serializing the entire state object on every update can be expensive in terms of both CPU and memory costs.
+If you are implementing an undo / redo feature, you likely want to take a snapshot of the application's state on every user edit. However, Vue's mutable reactivity system isn't best suited for this if the state tree is large, because serializing the entire state object on every update can be expensive in terms of both CPU and memory costs.
 
 [Immutable data structures](https://en.wikipedia.org/wiki/Persistent_data_structure) solve this by never mutating the state objects - instead, it creates new objects that share the same, unchanged parts with old ones. There are different ways of using immutable data in JavaScript, but we recommend using [Immer](https://immerjs.github.io/immer/) with Vue because it allows you to use immutable data while keeping the more ergonomic, mutable syntax.
 

src/tutorial/src/step-9/description.md

@@ -97,4 +97,4 @@ createApp({
 
 This is called a **lifecycle hook** - it allows us to register a callback to be called at certain times of the component's lifecycle. There are other hooks such as <span class="options-api">`created` and `updated`</span><span class="composition-api">`onUpdated` and `onUnmounted`</span>. Check out the <a target="_blank" href="/guide/essentials/lifecycle.html#lifecycle-diagram">Lifecycle Diagram</a> for more details.
 
-Now, try to add an <span class="options-api">`mounted`</span><span class="composition-api">`onMounted`</span> hook, access the `<p>` via <span class="options-api">`this.$refs.p`</span><span class="composition-api">`p.value`</span>, and perform some direct DOM operations on it (e.g. changing its `textContent`).
+Now, try to add <span class="options-api">a `mounted`</span><span class="composition-api">an `onMounted`</span> hook, access the `<p>` via <span class="options-api">`this.$refs.p`</span><span class="composition-api">`p.value`</span>, and perform some direct DOM operations on it (e.g. changing its `textContent`).