Use the more elegant way of querying the period's content nodes, now …

…that it's available

print/components/ProgramPeriod.vue

@@ -35,7 +35,7 @@ export default {
     await Promise.all([
       this.period.days().$loadItems(),
       this.period.scheduleEntries().$loadItems(),
-      this.$api.get().contentNodes({ period: this.period._meta.self }).$loadItems(),
+      this.period.contentNodes().$loadItems(),
     ])
 
     this.days = this.period.days().items