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

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