chore: fix typos (#852)

* chore: fix type ::tip to :::tip

* chore: fix nested typo

Author: pikax

src/guide/reactivity-fundamentals.md

@@ -73,15 +73,15 @@ When a ref is returned as a property on the render context (the object returned
 </script>
 ```
 
-::tip
+:::tip
   If you don't need to access the actual object instance, you can wrap it in a reactive:
 
   ```js
-  rested: reactive({
+  nested: reactive({
     count
   })
   ```
-::
+:::
 
 ### Access in Reactive Objects