parser: use `v fmt -w "$p.scanner.file_path"` in warns, easing fixing

vlib/v/parser/fn.v

@@ -552,7 +552,7 @@ fn (mut p Parser) fn_args() ([]table.Param, bool, bool) {
 			for p.tok.kind == .comma {
 				if !p.pref.is_fmt {
 					p.warn('`fn f(x, y Type)` syntax has been deprecated and will soon be removed. ' +
-						'Use `fn f(x Type, y Type)` instead. You can run `v fmt -w file.v` to automatically fix your code.')
+						'Use `fn f(x Type, y Type)` instead. You can run `v fmt -w "$p.scanner.file_path"` to automatically fix your code.')
 				}
 				p.next()
 				arg_pos << p.tok.position()