NEW YORK — Not five minutes into the second period Sunday afternoon at rollicking Madison Square Garden, the New York Rangers were scoring their second goal in a blink and Washington Capitals defenseman Vincent Iorio was sprawled along the boards, grasping his shoulder, prone on the ice. The Capitals, irate, protested. To win this first-round Stanley Cup playoff series, they will need an argument or two to fall their way. This one didn’t.