make statement more clearly (#32)

* make statement more clearly

It seems like the latter sentence has nothing to do with the previous sentence, thus they probably shouldn't be in the same paragraph.

* sync with English version

Author: zgayjjf

api.md

@@ -392,7 +392,9 @@ watchEffect(onInvalidate => {
 })
 ```
 
-We are registering the invalidation callback via a passed-in function instead of returning it from the callback (like React `useEffect`) because the return value is important for async error handling. It is very common for the effect function to be an async function when performing data fetching:
+We are registering the invalidation callback via a passed-in function instead of returning it from the callback (like React `useEffect`) because the return value is important for async error handling. 
+
+It is very common for the effect function to be an async function when performing data fetching:
 
 ```js
 const data = ref(null)

zh/api.md

@@ -393,7 +393,9 @@ watchEffect((onInvalidate) => {
 })
 ```
 
-我们之所以是通过传入一个函数去注册失效回调，而不是从回调返回它（如 React `useEffect` 中的方式），是因为返回值对于异步错误处理很重要。在执行数据请求时，副作用函数往往是一个异步函数：
+我们之所以是通过传入一个函数去注册失效回调，而不是从回调返回它（如 React `useEffect` 中的方式），是因为返回值对于异步错误处理很重要。
+
+在执行数据请求时，副作用函数往往是一个异步函数：
 
 ```js
 const data = ref(null)