docs(readme): replace pub.dartlang.org with pub.dev; normalize macOS; update Flutter docs link (#555)

Co-authored-by: jihanurrahman33 <[email protected]>

Author: jihanurrahman33

README.md

@@ -1,6 +1,6 @@
 # dart-sip-ua
 
-[![Financial Contributors on Open Collective](https://opencollective.com/flutter-webrtc/all/badge.svg?label=financial+contributors)](https://opencollective.com/flutter-webrtc) [![pub package](https://img.shields.io/pub/v/sip_ua.svg)](https://pub.dartlang.org/packages/sip_ua)  [![slack](https://img.shields.io/badge/join-us%20on%20slack-gray.svg?longCache=true&logo=slack&colorB=brightgreen)](https://join.slack.com/t/flutterwebrtc/shared_invite/zt-q83o7y1s-FExGLWEvtkPKM8ku_F8cEQ)
+[![Financial Contributors on Open Collective](https://opencollective.com/flutter-webrtc/all/badge.svg?label=financial+contributors)](https://opencollective.com/flutter-webrtc) [![pub package](https://img.shields.io/pub/v/sip_ua.svg)](https://pub.dev/packages/sip_ua)  [![slack](https://img.shields.io/badge/join-us%20on%20slack-gray.svg?longCache=true&logo=slack&colorB=brightgreen)](https://join.slack.com/t/flutterwebrtc/shared_invite/zt-q83o7y1s-FExGLWEvtkPKM8ku_F8cEQ)
 
 A dart-lang version of the SIP UA stack, ported from [JsSIP](https://github.com/versatica/JsSIP).