Photo courtesy of army.arch SPORTS: D.C. United takes on Seattle Sounders FC at RFK Stadium tonight in the U.S. Open Cup final. Tickets can still be had here for between $12-$30, plus fees. MUSIC: For an early start to your evening, head to the Kennedy Center’s Millennium Stage to hear award-winning Brazilian classical guitarist Joao Paulo Figueiro at 6 p.m. for free. LECTURE: It’s the first Wednesday of the month, so it’s time for…