Fixes broken link (dotnet#5107)

core/extensions/COMServerDemo/README.md

@@ -12,7 +12,7 @@ description: "A basic example of a managed COM server in .NET Core"
 
 # COM Server Demo
 
-This is a basic example of providing a managed COM server in .NET Core 3.1. Documentation on the inner workings of activation can be found [here](https://github.com/dotnet/core-setup/blob/main/Documentation/design-docs/COM-activation.md).
+This is a basic example of providing a managed COM server in .NET Core 3.1. Documentation on the inner workings of activation can be found [here](https://github.com/dotnet/runtime/blob/main/docs/design/features/COM-activation.md).
 
 ## Key Features