cmd/present: skip the "golang.org" directory

Change-Id: I732194c22062c77c431dcec57f9ef832f1dc1afe
Signed-off-by: Shenghou Ma <minux@golang.org>
Reviewed-on: https://go-review.googlesource.com/1512
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>