Fix utf8-bom test

The BOM was accidentally removed in rust-lang#57108

Author: ehuss

tests/ui/utf8-bom.rs

@@ -1,6 +1,4 @@
+// This file has utf-8 BOM, it should be compiled normally without error.
 //@ run-pass
-//
-
-// This file has utf-8 BOM, it should be compiled normally without error.
 
 pub fn main() {}