It's supposed to be there -- it indicates a one-tuple. (Otherwise it would just be a parenthesis wrapped expression.)
let x = (32); // type i32
let y = (32,); // type (i32,)
...as for actually interpretting what it means, the FnMut traits are defined in terms of tuples currently, so that's a closure that takes a single argument, which is a reference to a two-tuple.