修复 theme-change 示例js环境报错的问题

pages/API/theme-change/theme-change.uvue

@@ -46,13 +46,13 @@
     methods: {
       bindOsThemeChange(): number {
         //注册osTheme变化监听
-        return uni.onOsThemeChange(function(res: OsThemeChangeResult) {
+        return uni.onOsThemeChange((res: OsThemeChangeResult)=> {
           this.osTheme = res.osTheme
         })
       },
       bindAppThemeChange(): number {
         //注册appTheme变化监听
-        return uni.onAppThemeChange(function(res: AppThemeChangeResult) {
+        return uni.onAppThemeChange((res: AppThemeChangeResult) => {
           this.appTheme = res.appTheme
         })
       },