add launcher icon to flutter app code example

Example 1: flutter launcher icon generate

dev_dependencies:
  flutter_launcher_icons: ^0.8.1
flutter_icons:
  android: true
  ios: true
  image_path: "assets/icon/icon.png"
  
 then
 cmd ->flutter pub run flutter_launcher_icons:main

Example 2: flutter app icon

dev_dependencies:
  flutter_launcher_icons: "^0.7.3"

flutter_icons:
  android: "launcher_icon"
  ios: true
  image_path: "assets/icon/icon.png"