docs(demos): fix date-time-picker model value

Closes #423

Author: Matt Lewis

demos/demo-modules/demo-utils/date-time-picker.component.ts

@@ -102,6 +102,7 @@ export class DateTimePickerComponent implements ControlValueAccessor {
       ),
       this.dateStruct.year
     );
+    this.writeValue(newDate);
     this.onChangeCallback(newDate);
   }
 
@@ -113,6 +114,7 @@ export class DateTimePickerComponent implements ControlValueAccessor {
       ),
       this.timeStruct.hour
     );
+    this.writeValue(newDate);
     this.onChangeCallback(newDate);
   }
 }