cmd/compile/internal/scanner: report correct directive string (fix bu…

…ild)

Change-Id: I01b244e97e4140545a46b3d494489a30126c2139
Reviewed-on: https://go-review.googlesource.com/c/go/+/222257
Run-TryBot: Robert Griesemer <gri@golang.org>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>

src/cmd/compile/internal/syntax/scanner.go

@@ -704,7 +704,7 @@ func (s *scanner) rawString() {
 }
 
 func (s *scanner) comment(text string) {
-	s.errorAtf(0, text)
+	s.errorAtf(0, "%s", text)
 }
 
 func (s *scanner) skipLine() {