Update README to reflect correct dependency format for flutter_poolakey

[OmidHaqi]

Summary of Change:

The dependency declaration in the README.md was changed from:

dependencies:
  flutter_poolakey: ^2.2.0-1.0.0-alpha01

to:

dependencies:
  flutter_poolakey:
    git:
      url: https://github.com/cafebazaar/flutter_poolakey.git
      ref: 2.2.0-1.0.0-alpha01

Reason:

The version 2.2.0-1.0.0-alpha01 is not available on pub.dev, so users need to fetch it directly from the GitHub repository. This update helps developers add the correct dependency and avoid confusion or errors when running flutter pub get.