Fix error index test

Author: estebank

src/librustc_error_codes/error_codes/E0746.md

@@ -2,7 +2,8 @@ Return types cannot be `dyn Trait`s as they must be `Sized`.
 
 Erroneous code example:
 
-```compile_fail,E0746
+```compile_fail,E0277
+# // FIXME: after E0746 is in beta, change the above
 trait T {
     fn bar(&self);
 }