Update reactivity-fundamentals.md (#1608)

In the DOM Update Timing section's example, it seems we should use state.count++(which has been shown above) instead of count.value++(which comes later).

Author: WangNingning1994

src/guide/essentials/reactivity-fundamentals.md

@@ -224,7 +224,7 @@ To wait for the DOM update to complete after a state change, you can use the [ne
 import { nextTick } from 'vue'
 
 function increment() {
-  count.value++
+  state.count++
   nextTick(() => {
     // access updated DOM
   })