Fix metadata_lib_filename_form

Author: YC

compiler/rustc_error_messages/locales/en-US/metadata.ftl

@@ -275,7 +275,7 @@ metadata_crate_location_unknown_type =
     extern location for {$crate_name} is of an unknown type: {$path}
 
 metadata_lib_filename_form =
-    file name should be lib*.rlib or {dll_prefix}*.{dll_suffix}
+    file name should be lib*.rlib or {$dll_prefix}*.{$dll_suffix}
 
 metadata_multiple_import_name_type =
     multiple `import_name_type` arguments in a single `#[link]` attribute