Fix compile-fail/cfg-empty-codemap.rs test

Author: pietroalbini

src/test/compile-fail/cfg-empty-codemap.rs

@@ -12,7 +12,7 @@
 
 // compile-flags: --cfg ""
 
-// error-pattern: expected identifier, found
+// error-pattern: invalid `--cfg` argument
 
 pub fn main() {
 }