fix: ref example (vuejs#2570)

Author: jacekkarczmarczyk

src/api/reactivity-core.md

@@ -35,7 +35,7 @@ Takes an inner value and returns a reactive and mutable ref object, which has a
   const count = ref(0)
   console.log(count.value) // 0
 
-  count.value++
+  count.value = 1
   console.log(count.value) // 1
   ```