change app publish flutter code example

Example 1: create signingConfigs flutter macos

keytool -genkey -v -keystore ~/key.jks -keyalg RSA -keysize 2048 -validity 10000 -alias key

Example 2: flutter publish app

keytool -genkey -v -keystore c:\Users\USER_NAME\key.jks -storetype JKS -keyalg RSA -keysize 2048 -validity 10000 -alias key