Change backups submit button to fix translation issues

Reviewed-by: James Valleroy <jvalleroy@mailbox.org>

plinth/modules/backups/templates/backups_schedule.html

@@ -16,6 +16,6 @@
     {{ form|bootstrap }}
 
     <input type="submit" class="btn btn-primary"
-           value="{% trans 'Update' %}" />
+           value="{% trans 'Submit' %}" />
   </form>
 {% endblock %}