go.tools/cmd/present: update doc
go.tools/cmd/present: update doc

LGTM=adg
R=golang-codereviews, dan.kortschak, gobot, adg
CC=golang-codereviews
https://codereview.appspot.com/106860043

Committer: Andrew Gerrand <395a7d33bec8475c9b83b7d440f141bcbd994aa5@golang.org>

bryanxu/go-zh.tools,d0f/go-zh.tools,skyportsystems/go.tools,8limbs/gotools,d0f/go-zh.tools,d0f/go-zh.tools,8limbs/gotools,bryanxu/go-zh.tools,skyportsystems/go.tools,skyportsystems/go.tools,8limbs/gotools,skyportsystems/go.tools,webfd/go-zh.tools,bryanxu/go-zh.tools,webfd/go-zh.tools,d0f/go-zh.tools,webfd/go-zh.tools,d0f/go-zh.tools,webfd/go-zh.tools,webfd/go-zh.tools,bryanxu/go-zh.tools,bryanxu/go-zh.tools,skyportsystems/go.tools