Fix CONTRIBUTING.md .NET 5 SDK link (#17161)

CONTRIBUTING.md

@@ -9,7 +9,7 @@
 - Install VS 2019 (Community or higher) and make sure you have the latest updates (https://www.visualstudio.com/).
   - Need at least .NET Framework 4.6.1 and 4.7 development tools
 - Install the **.NET Core cross-platform development** workloads in VisualStudio
-- Install **.NET Core 5.0.100 SDK** for your specific platform. (or a higher version within the 5.0.*** band)  (https://dotnet.microsoft.com/download/dotnet-core/3.1)
+- Install **.NET Core 5.0.100 SDK** for your specific platform. (or a higher version within the 5.0.*** band)  (https://dotnet.microsoft.com/download/dotnet-core/5.0)
 - Install the latest version of git (https://git-scm.com/downloads)
 - Install [NodeJS](https://nodejs.org/en/) (14.x.x) if you plan to use [C# code generation](https://github.com/Azure/autorest.csharp).