Correct error message that changed.

Author: nikomatsakis

src/test/compile-fail/unboxed-closures-type-mismatch.rs

@@ -14,6 +14,6 @@ use std::ops::FnMut;
 
 pub fn main() {
     let mut f = |&mut: x: int, y: int| -> int { x + y };
-    let z = f.call_mut((1u, 2));    //~ ERROR type mismatch
+    let z = f.call_mut((1u, 2));    //~ ERROR not implemented
     println!("{}", z);
 }