Add vertical spacing above set channel h1

To make it match other pages which use the legend
as the h1.

app/templates/views/service-settings/service-set-broadcast-channel.html

@@ -18,6 +18,7 @@
       {% call form_wrapper() %}
         {{ form.channel(param_extensions={
           'fieldset': {
+            'classes': 'govuk-!-margin-top-3',
             'legend': {
               'isPageHeading': True,
               'classes': 'govuk-fieldset__legend--l'