codelab/index.html "os" needs to be imported because os.Error is used almost immediately

Also, calling fmt, ioutil and os "builtin" is probably confusing.

R=adg
CC=golang-dev
https://golang.org/cl/965045

doc/codelab/wiki/index.html

@@ -58,13 +58,15 @@ package main
 import (
 	"fmt"
 	"io/ioutil"
+	"os"
 )
 </pre>
 
 <p>
-Both <code>fmt</code> and <code>ioutil</code> are built-in packages that 
+We import the <code>fmt</code>, <code>ioutil</code> and <code>os</code>
-we'll be using. Later, as we implement additional functionality, we will add 
+packages from the Go standard library. Later, as we implement additional
-more packages to this <code>import</code> declaration.
+functionality, we will add more packages to this <code>import</code>
+declaration.
 </p>
 
 <h2>Data Structures</h2>
@@ -304,6 +306,7 @@ import (
 	"fmt"
 	<b>"http"</b>
 	"io/ioutil"
+	"os"
 )
 </pre>