Firefighters rescued a man from the second-floor scaffolding of a Beacon Hill building under renovation on Wednesday afternoon, the Boston Fire Department said. The man was working at the building on Charles and Revere streets when he injured his knee and was unable to lower himself to the ground, said department spokesman Steve MacDonald. Fire officials used a tall ladder to place him on a stretcher and move him to the ground, MacDonald said. He was taken to a local hospital. The cause of the injury was not immediately clear.