fix: correct a couple of typos in reactivity-fundamentals.md (#853)

Co-authored-by: Natalia Tepluhina <[email protected]>

Author: skirtles-code

src/guide/reactivity-fundamentals.md

@@ -74,13 +74,13 @@ When a ref is returned as a property on the render context (the object returned
 ```
 
 :::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
+})
+```
 :::
 
 ### Access in Reactive Objects