Update issue-14084.rs test to reflect changes to error output.

Author: pnkfelix

src/test/compile-fail/issue-14084.rs

@@ -9,8 +9,10 @@
 // except according to those terms.
 
 #![feature(box_syntax)]
+#![feature(placement_in_syntax)]
 
 fn main() {
     box ( () ) 0;
-    //~^ ERROR: only the exchange heap is currently supported
+    //~^ ERROR: the trait `core::ops::Placer<_>` is not implemented
+    //~| ERROR: the trait `core::ops::Placer<_>` is not implemented
 }