Change NuGet badge to point to pre-release version (#27)

Author: joseharriaga

README.md

@@ -1,6 +1,6 @@
 # OpenAI .NET API library
 
-[![NuGet version](https://img.shields.io/nuget/v/openai.svg)](https://www.nuget.org/packages/OpenAI/)
+[![NuGet version](https://img.shields.io/nuget/vpre/openai.svg)](https://www.nuget.org/packages/OpenAI/)
 
 The OpenAI .NET library provides convenient access to the OpenAI REST API from .NET applications.