fix: update license reference (#199)

Author: emmanuel-ferdman

README.md

@@ -141,4 +141,4 @@ This project welcomes contributions and suggestions.
 Do you want to contribute to the project? Find out how [here](CONTRIBUTING.md).
 
 ## License
-[Modified Apache 2.0 (Section 6)](https://github.com/LEGO/AsyncAPI.NET/blob/main/LICENSE)
+[Modified Apache 2.0 (Section 6)](https://github.com/LEGO/AsyncAPI.NET/blob/main/LICENSE.txt)