Update Startup.cs

`builder.AddUserSecrets();` is deprecated and will not be available in ASP.NET Core 2.0.

src/Services/Identity/Identity.API/Startup.cs

@@ -37,7 +37,7 @@ namespace eShopOnContainers.Identity
             if (env.IsDevelopment())
             {
                 // For more details on using the user secret store see http://go.microsoft.com/fwlink/?LinkID=532709
-                builder.AddUserSecrets();
+                builder.AddUserSecrets<Startup>();
             }
 
             builder.AddEnvironmentVariables();