Readme: update status badge

Author: swharden

README.md

@@ -1,6 +1,6 @@
 # Spectrogram
 
-[![](https://img.shields.io/azure-devops/build/swharden/swharden/9?label=Build&logo=azure%20pipelines)](https://dev.azure.com/swharden/swharden/_build/latest?definitionId=9&branchName=master)
+[![CI](https://github.com/swharden/Spectrogram/actions/workflows/ci.yaml/badge.svg)](https://github.com/swharden/Spectrogram/actions/workflows/ci.yaml)
 [![Nuget](https://img.shields.io/nuget/v/Spectrogram?label=NuGet&logo=nuget)](https://www.nuget.org/packages/Spectrogram/)
 
 **Spectrogram** is a .NET library for creating spectrograms from pre-recorded signals or live audio from the sound card.  Spectrogram uses FFT algorithms and window functions provided by the [FftSharp](https://github.com/swharden/FftSharp) project, and it targets .NET Standard so it can be used in .NET Framework and .NET Core projects.