Update checkboxes data-module to govuk-checkboxes

This change is required due to new approach to namespacing[0] in V3.0
of the GOV.UK Design System

[0] alphagov/govuk-frontend#1443

lib/govuk_design_system_formbuilder/containers/check_boxes.rb

@@ -7,7 +7,7 @@ def initialize(builder, small:)
       end
 
       def html
-        @builder.content_tag('div', class: check_boxes_classes, data: { module: 'checkboxes' }) do
+        @builder.content_tag('div', class: check_boxes_classes, data: { module: 'govuk-checkboxes' }) do
           yield
         end
       end

spec/govuk_design_system_formbuilder/builder/check_boxes_spec.rb

@@ -53,8 +53,10 @@
 
     describe 'check boxes' do
       specify 'output should contain the correct number of check boxes' do
-        expect(subject).to have_tag('input', count: projects.size, with: { type: 'checkbox' })
-        expect(subject).to have_tag('label', count: projects.size)
+        expect(subject).to have_tag('div', with: { 'data-module' => 'govuk-checkboxes' }) do |cb|
+          expect(cb).to have_tag('input', count: projects.size, with: { type: 'checkbox' })
+          expect(cb).to have_tag('label', count: projects.size)
+        end
       end
 
       context 'check box size' do
@@ -173,7 +175,7 @@
       end
 
       specify 'output should contain check boxes' do
-        expect(subject).to have_tag('div', with: { class: 'govuk-checkboxes', 'data-module' => 'checkboxes' }) do
+        expect(subject).to have_tag('div', with: { class: 'govuk-checkboxes', 'data-module' => 'govuk-checkboxes' }) do
           expect(subject).to have_tag('input', with: { type: 'checkbox' }, count: 3)
         end
       end