tutorial: a couple of minor fixes.

Fixes #1333.

R=rsc, gri
CC=golang-dev
https://golang.org/cl/3556041

Author: robpike

doc/go_tutorial.html

@@ -649,8 +649,8 @@ <h2>An I/O Package</h2>
 13    func main() {
 14        hello := []byte(&quot;hello, world\n&quot;)
 15        file.Stdout.Write(hello)
-16        file, err := file.Open(&quot;/does/not/exist&quot;,  0,  0)
-17        if file == nil {
+16        f, err := file.Open(&quot;/does/not/exist&quot;,  0,  0)
+17        if f == nil {
 18            fmt.Printf(&quot;can't open file; err=%s\n&quot;,  err.String())
 19            os.Exit(1)
 20        }

doc/progs/helloworld3.go

@@ -13,8 +13,8 @@ import (
 func main() {
 	hello := []byte("hello, world\n")
 	file.Stdout.Write(hello)
-	file, err := file.Open("/does/not/exist",  0,  0)
-	if file == nil {
+	f, err := file.Open("/does/not/exist",  0,  0)
+	if f == nil {
 		fmt.Printf("can't open file; err=%s\n",  err.String())
 		os.Exit(1)
 	}

doc/progs/sortmain.go

@@ -6,7 +6,7 @@ package main
 
 import (
 	"fmt"
-	"sort"
+	"./sort"
 )
 
 func ints() {