Switch README version badge to update quicker

Author: alexrudall

README.md

@@ -1,6 +1,6 @@
 # Ruby OpenAI
 
-[![Gem Version](https://badge.fury.io/rb/ruby-openai.svg)](https://badge.fury.io/rb/ruby-openai)
+[![Gem Version](https://img.shields.io/gem/v/ruby-openai.svg)](https://rubygems.org/gems/ruby-openai)
 [![GitHub license](https://img.shields.io/badge/license-MIT-blue.svg)](https://github.com/alexrudall/ruby-openai/blob/main/LICENSE.txt)
 [![CircleCI Build Status](https://circleci.com/gh/alexrudall/ruby-openai.svg?style=shield)](https://circleci.com/gh/alexrudall/ruby-openai)