docs: Update badge link for Pub Version in README (#308)

Ensures that tapping on the Pub version shield links to pub.dev

Author: miquelbeltran

README.md

@@ -1,6 +1,6 @@
 # raygun4flutter
 
-![Pub Version](https://img.shields.io/pub/v/raygun4flutter)
+[![Pub Version](https://img.shields.io/pub/v/raygun4flutter)](https://pub.dev/packages/raygun4flutter)
 [![CI](https://github.com/MindscapeHQ/raygun4flutter/actions/workflows/main.yml/badge.svg)](https://github.com/MindscapeHQ/raygun4flutter/actions/workflows/main.yml)
 
 The world's best Flutter Crash Reporting solution.