Fixed link to Azure SDK Design Guidelines for .NET (#15210)

sdk/core/Azure.Core/README.md

@@ -1,7 +1,7 @@
 # Azure Core shared client library for .NET
 
 Azure.Core provides shared primitives, abstractions, and helpers for modern .NET Azure SDK client libraries. 
-These libraries follow the [Azure SDK Design Guidelines for .NET](https://azuresdkspecs.z5.web.core.windows.net/DotNetSpec.html) 
+These libraries follow the [Azure SDK Design Guidelines for .NET](https://azure.github.io/azure-sdk/dotnet_introduction.html) 
 and can be easily identified by package and namespaces names starting with 'Azure', e.g. ```Azure.Storage.Blobs```. 
 A more complete list of client libraries using Azure.Core can be found [here](https://github.com/Azure/azure-sdk-for-net#core-services).