Fix new end-to-end test

This fix is based on the following comment in the MR that added
the new test:
https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/15857

qa/qa/specs/features/ee/browser_ui/2_plan/custom_email/custom_email_spec.rb

@@ -7,7 +7,7 @@ module QA
     describe 'Custom email' do
       before do
         Runtime::Browser.visit(:gitlab, Page::Main::Login)
-        Page::Main::Login.perform(&:sign_in_using_credentials)
+        Page::Main::Login.perform(&:sign_in_using_admin_credentials)
         Page::Main::Menu.perform(&:click_admin_area)
         Page::Admin::Menu.perform(&:go_to_preferences_settings)
       end