Make sure Identity still uses Bootstrap4 (#88)

Author: AndriySvyryd

docs/4. Add auth features.md

@@ -338,6 +338,7 @@ In this module we're going to add the capability for users to register and sign-
         options.Password.RequireUppercase = false;
         options.Password.RequireNonAlphanumeric = false;
     })
+    .AddDefaultUI(UIFramework.Bootstrap4)
     .AddEntityFrameworkStores<IdentityDbContext>()
     .AddClaimsPrincipalFactory<ClaimsPrincipalFactory>();
     ```