Fix link to sample on GitHub (#17922)

* Fix link to sample on GitHub

Fixes #17918

* Use samples in v3.1.3 branch instead of Master

Author: mjrousos

aspnetcore/security/authorization/iauthorizationpolicyprovider.md

@@ -19,7 +19,7 @@ Examples of scenarios where a custom [IAuthorizationPolicyProvider](/dotnet/api/
 * Using a large range of policies (for different room numbers or ages, for example), so it doesn't make sense to add each individual authorization policy with an `AuthorizationOptions.AddPolicy` call.
 * Creating policies at runtime based on information in an external data source (like a database) or determining authorization requirements dynamically through another mechanism.
 
-[View or download sample code](https://github.com/dotnet/AspNetCore/tree/release/2.2/src/Security/samples/CustomPolicyProvider) from the [AspNetCore GitHub repository](https://github.com/dotnet/AspNetCore). Download the dotnet/AspNetCore repository ZIP file. Unzip the file. Navigate to the *src/Security/samples/CustomPolicyProvider* project folder.
+[View or download sample code](https://github.com/dotnet/aspnetcore/tree/v3.1.3/src/Security/samples/CustomPolicyProvider) from the [AspNetCore GitHub repository](https://github.com/dotnet/AspNetCore). Download the dotnet/AspNetCore repository ZIP file. Unzip the file. Navigate to the *src/Security/samples/CustomPolicyProvider* project folder.
 
 ## Customize policy retrieval
 
@@ -178,4 +178,4 @@ To use custom policies from an `IAuthorizationPolicyProvider`, you must:
 services.AddSingleton<IAuthorizationPolicyProvider, MinimumAgePolicyProvider>();
 ```
 
-A complete custom `IAuthorizationPolicyProvider` sample is available in the [dotnet/aspnetcore GitHub repository](https://github.com/dotnet/aspnetcore/tree/ea555458dc61e04314598c25b3ab8c56362a5123/src/Security/samples/CustomPolicyProvider).
+A complete custom `IAuthorizationPolicyProvider` sample is available in the [dotnet/aspnetcore GitHub repository](https://github.com/dotnet/aspnetcore/tree/v3.1.3/src/Security/samples/CustomPolicyProvider).