Update README.md (#35)

- Fix the branch name from `master` to `main`.

Author: justinyoo

README.md

@@ -2,7 +2,7 @@
 
 ### Practical lessons teaching you how to build Generative AI applications in .NET
 
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+[![GitHub license](https://img.shields.io/github/license/microsoft/Generative-AI-For-Beginners-Dotnet.svg)](https://github.com/microsoft/Generative-AI-For-Beginners-Dotnet/blob/main/LICENSE)
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