os: use Println instead of Printf in example

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>