doc: update document what import pinyin method in v4

Author: hotoo

docs/EXAMPLE.md

@@ -2,7 +2,7 @@
 
 ```jsx
 import React, { useState, useEffect } from 'react';
-import pinyin from '../lib/umd/pinyin';
+import { pinyin } from '../lib/umd/pinyin';
 import { JsonViewer } from '@textea/json-viewer';
 
 export default function() {

docs/api/v4/index.en-US.md

@@ -52,7 +52,7 @@ npm install pinyin --save
 for developer:
 
 ```typescript
-import pinyin from "pinyin";
+import { pinyin } from "pinyin";
 
 console.log(pinyin("中心"));    // [ [ 'zhōng' ], [ 'xīn' ] ]
 

docs/api/v4/index.ko-KR.md

@@ -52,7 +52,7 @@ npm install pinyin --save
 개발자：
 
 ```typescript
-import pinyin from "pinyin";
+import { pinyin } from "pinyin";
 
 console.log(pinyin("中心"));    // [ [ 'zhōng' ], [ 'xīn' ] ]
 

docs/api/v4/index.md

@@ -61,7 +61,7 @@ npm install pinyin --save
 开发者：
 
 ```typescript
-import pinyin from "pinyin";
+import { pinyin } from "pinyin";
 
 console.log(pinyin("中心"));    // [ [ 'zhōng' ], [ 'xīn' ] ]
 

docs/index.en-US.md

@@ -52,7 +52,7 @@ npm install pinyin --save
 for developer:
 
 ```typescript
-import pinyin from "pinyin";
+import { pinyin } from "pinyin";
 
 console.log(pinyin("中心"));    // [ [ 'zhōng' ], [ 'xīn' ] ]
 

docs/index.ko-KR.md

@@ -52,7 +52,7 @@ npm install pinyin --save
 개발자：
 
 ```typescript
-import pinyin from "pinyin";
+import { pinyin } from "pinyin";
 
 console.log(pinyin("中心"));    // [ [ 'zhōng' ], [ 'xīn' ] ]
 

docs/index.md

@@ -53,7 +53,7 @@ npm install pinyin --save
 开发者：
 
 ```typescript
-import pinyin from "pinyin";
+import { pinyin } from "pinyin";
 
 console.log(pinyin("中心"));    // [ [ 'zhōng' ], [ 'xīn' ] ]