You may think you know Washington Irving’s “Legend of Sleepy Hollow,” but the original is nothing like the new musical based on that book at Signature Theatre. “The Hollow,” with book by Hunter Foster and music and lyrics by Matt Conner, places the action in upstate New York and uses many of the same characters Irving did: the schoolteacher Ichabod Crane, the lovely Katrina Van Tassel, her fiance, Brom Bones, and, of course, the Headless Horseman.