go.tools/cmd/present: update docs after move
go.tools/cmd/present: update docs after move

LGTM=bradfitz
R=golang-codereviews, bradfitz
CC=golang-codereviews
https://codereview.appspot.com/99060043

t-yuki/go.tools.old,t-yuki/go.tools.old,t-yuki/go.tools.old