Increase prominence of vaccine choice heading in clinic booking

As requested in PR #223

Author: malross

app/views/book-into-a-clinic/form/vaccination-choice.njk

@@ -46,7 +46,8 @@
   {{ checkboxes({
     fieldset: {
       legend: {
-        text: __("clinicBooking.vaccinationChoice.vaccinations.label", firstName)
+        text: __("clinicBooking.vaccinationChoice.vaccinations.label", firstName),
+        size: "m"
       }
     },
     hint: { text: __("clinicBooking.vaccinationChoice.vaccinations.hint") },