benchcmp: Fix invocation instructions.
benchcmp: Fix invocation instructions.

`go test` takes -run and -bench; the -test.run and -test.bench flags
are only for the test binary itself.

LGTM=adg
R=adg
CC=golang-codereviews
https://codereview.appspot.com/113390043

karmakaze/go.tools,karmakaze/go.tools,karmakaze/go.tools,karmakaze/go.tools,karmakaze/go.tools