Update build status badge to branch '3.x' (#1581)

Author: bart-vmware

README.md

@@ -1,6 +1,6 @@
 # Steeltoe .NET Open Source Software
 
-[![Build Status](https://dev.azure.com/SteeltoeOSS/Steeltoe/_apis/build/status/Steeltoe.All?branchName=main)](https://dev.azure.com/SteeltoeOSS/Steeltoe/_build/latest?definitionId=4&branchName=main)
+[![Build Status](https://dev.azure.com/SteeltoeOSS/Steeltoe/_apis/build/status/Steeltoe.All?branchName=3.x)](https://dev.azure.com/SteeltoeOSS/Steeltoe/_build/latest?definitionId=4&branchName=3.x)
 
 ## Why Steeltoe?