docs: update components Text's example code

docs/components/base/text.md

@@ -25,44 +25,44 @@ sidebar_label: Text
 ###### 示例：
 ```jsx
 import Taro, { Component } from '@tarojs/taro'
-import { View, Text } from '@tarojs/components'
+import { View, Text, Button } from '@tarojs/components'
 
 export default class PageView extends Component {
-  constructor () {
-    super(...arguments)
-
-    this.state = {
-      contents = []
-    }
+  state = {
+    contents: [],
+    contentsLen: 0
   }
 
-  add = e => {
-    const cot = this.state.contents
-    cot.push({text: 'hello world'})
-
-    this.setState(() => {
-      return {contents: cot}
+  add = () => {
+    this.setState(prev => {
+      const cot = prev.contents.slice()
+      cot.push({ text: 'hello world' })
+      return {
+        contents: cot,
+        contentsLen: cot.length
+      }
     })
   }
 
-  remove = e => {
-    const cot = this.state.contents
-    cot.pop()
-    this.setState(() => {
-      return {contents: cot}
+  remove = () => {
+    this.setState(prev => {
+      const cot = prev.contents.slice()
+      cot.pop()
+      return {
+        contents: cot,
+        contentsLen: cot.length
+      }
     })
   }
 
   render () {
     return (
       <View className='container'>
-        {this.state.contents.map(item => {
-          return (
-            <Text>{item.text}</Text>
-          )
-        })}
+        {this.state.contents.map((item, index) => (
+          <Text key={index}>{item.text}</Text>
+        ))}
         <Button className='btn-max-w button_style' plain type='default' onClick={this.add}>add line</Button>
-        <Button className='btn-max-w button_style' plain type='default' disabled={this.state.contents.length ? false:true} onClick={this.remove}>remove line</Button>
+        <Button className='btn-max-w button_style' plain type='default' disabled={this.state.contentsLen ? false : true} onClick={this.remove}>remove line</Button>
       </View>
     )
   }