doc/codelab: correct typo.

s/Sprintf/Fprintf/

R=golang-dev, rsc
CC=adg, golang-dev
https://golang.org/cl/4519053

doc/codelab/wiki/wiki.html

@@ -428,7 +428,7 @@ This will prevent user data from corrupting the form HTML.
 </p>
 
 <p>
-Now that we've removed the <code>fmt.Sprintf</code> statement, we can remove
+Now that we've removed the <code>fmt.Fprintf</code> statement, we can remove
 <code>"fmt"</code> from the <code>import</code> list.
 </p>