os: add example for IsNotExist

Show usage of os.IsNotExist in an example.

Change-Id: I5306ea06c370099de5b02668dfa02b87b0c2beac
Reviewed-on: https://go-review.googlesource.com/25571
Reviewed-by: Russ Cox <rsc@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/os/example_test.go

@@ -52,3 +52,12 @@ func ExampleFileMode() {
 		fmt.Println("named pipe")
 	}
 }
+
+func ExampleIsNotExist() {
+	filename := "a-nonexistent-file"
+	if _, err := os.Stat(filename); os.IsNotExist(err) {
+		fmt.Printf("file does not exist")
+	}
+	// Output:
+	// file does not exist
+}