md4: move the example to the right directory

Mistake from https://golang.org/cl/49030 (noted in comments after submission)

Change-Id: Id1c94e875487f380ab04d9797259243f31fe72c8
Reviewed-on: https://go-review.googlesource.com/50390
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Dmitri Shuralyov <shurcool@gmail.com>