fix: correct a couple of typos in reactivity-fundamentals.md

vuejs/docs@f9828e0#diff-55ff289006ef954900bf36417b8e59c1586919dd6ca8d8047d053517cacb8a97

Author: naokie

src/guide/reactivity-fundamentals.md

@@ -74,13 +74,13 @@ ref がレンダーコンテキスト(render contenxt - [setup()](composition-ap
 ```
 
 :::tip
-  If you don't need to access the actual object instance, you can wrap it in a reactive:
+If you don't need to access the actual object instance, you can wrap it in a `reactive`:
 
-  ```js
-  nested: reactive({
-    count
-  })
-  ```
+```js
+nested: reactive({
+  count
+})
+```
 :::
 
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