os: use Println instead of Printf in example

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It should use Println for consistency with other examples.

Change-Id: I49bd1652f9449fcbdd79c6b689c123090972aab3
Reviewed-on: https://go-review.googlesource.com/127836
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Reviewed-by: Tobias Klauser <tobias.klauser@gmail.com>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/os/example_test.go

@@ -70,7 +70,7 @@ func ExampleFileMode() {
 func ExampleIsNotExist() {
 	filename := "a-nonexistent-file"
 	if _, err := os.Stat(filename); os.IsNotExist(err) {
-		fmt.Printf("file does not exist")
+		fmt.Println("file does not exist")
 	}
 	// Output:
 	// file does not exist