os: correct func name in WriteFile godoc comment

os: correct func name in WriteFile godoc comment

Change-Id: Ideb70ce04f49ff676c20c2e1f0b43f1d7a6665dc
Reviewed-on: https://go-review.googlesource.com/c/go/+/474355
Reviewed-by: Rob Pike <r@golang.org>
TryBot-Result: Gopher Robot <gobot@golang.org>
Auto-Submit: Tobias Klauser <tobias.klauser@gmail.com>
Reviewed-by: Carlos Amedee <carlos@golang.org>
Reviewed-by: Ian Lance Taylor <iant@google.com>
Run-TryBot: Tobias Klauser <tobias.klauser@gmail.com>