testsuite: Update error message, unbreak build

src/test/compile-fail/assign-to-method.rs

@@ -29,5 +29,4 @@ fn cat(in_x : uint, in_y : int) -> cat {
 fn main() {
   let nyan : cat = cat(52u, 99);
   nyan.speak = fn@() { debug!("meow"); }; //~ ERROR attempted to take value of method
-  //~^ ERROR mismatched types
 }