Fix broken spec in help_controller_spec.rb

The master branch started failing because
e355afb4 deleted
`doc/user/project/img/labels_key_value_v12_1.png`. To make the build
pass, use another image for now.

Closes https://gitlab.com/gitlab-org/gitlab/issues/199976

spec/controllers/help_controller_spec.rb

@@ -111,7 +111,7 @@ describe HelpController do
         it 'renders the raw file' do
           get :show,
               params: {
-                path: 'user/project/img/labels_default_v12_1'
+                path: 'integration/img/gitlab_app'
               },
               format: :png
           expect(response).to be_successful