update one more test to use assertion IsInstanceOf

Author: Meaghan Lewis

test/UnitTests/GitHub.App/ViewModels/Dialog/GitHubDialogWindowViewModelTests.cs

@@ -54,8 +54,8 @@ public async Task ShowsLoginDialogWhenNoConnectionsAvailable()
 
                 await target.StartWithConnection(content);
 
-                Assert.IsAssignableFrom<ILoginViewModel>(target.Content);
-            }
+				Assert.That(target.Content, Is.InstanceOf<ILoginViewModel>());
+			}
 
             [Test]
             public async Task ShowsContentWhenConnectionAvailable()