Fix class modifier (dotnet#4261)

aspnetcore/security/authorization/resourcebased.md

@@ -77,7 +77,7 @@ Writing a handler for resource based authorization is not that much different to
 ```csharp
 public class DocumentAuthorizationHandler : AuthorizationHandler<MyRequirement, Document>
 {
-    public override Task HandleRequirementAsync(AuthorizationHandlerContext context,
+    protected override Task HandleRequirementAsync(AuthorizationHandlerContext context,
                                                 MyRequirement requirement,
                                                 Document resource)
     {