doc: fix fmt usage in code.html

Fixes #25876

Change-Id: I149dbb2afef13cc13fb5722d930377ed918d7e3c
Reviewed-on: https://go-review.googlesource.com/118717Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

doc/code.html

@@ -240,7 +240,7 @@ package main
 import "fmt"
 
 func main() {
-	fmt.Printf("Hello, world.\n")
+	fmt.Println("Hello, world.")
 }
 </pre>
 
@@ -395,7 +395,7 @@ import (
 )
 
 func main() {
-	fmt.Printf(stringutil.Reverse("!oG ,olleH"))
+	fmt.Println(stringutil.Reverse("!oG ,olleH"))
 }
 </pre>