Change header text in groups and headings section

The text "Groups and headings" might cause users to think both groups
and headings have the same hierarchy, and wonder whether what they
immediately see are groups or headings.

Using "Heading groups" we make it clear that what comes immediately is a
group, and then we see each group has headings.

spec/system/admin/budgets_wizard/groups_spec.rb

@@ -114,7 +114,7 @@ describe "Budgets wizard, groups step", :admin do
 
       visit admin_budget_path(budget)
 
-      within "section", text: "Groups and headings" do
+      within "section", text: "Heading groups" do
         expect(page).to have_css "h4", exact_text: "Group without typos"
       end
     end