go.tools/cover: clean up usage information
go.tools/cover: clean up usage information

R=golang-dev, r, minux.ma
CC=golang-dev
https://codereview.appspot.com/13532052

Committer: Rob Pike <4dc7c9ec434ed06502767136789763ec11d2c4b7@golang.org>

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