cmd/gomote: make it clearer how to list buildlet types
cmd/gomote: make it clearer how to list buildlet types

Fixes golang/go#20151

Change-Id: I2933b8d3d00b8765a1b5c7505e71d901c88e4008
Reviewed-on: https://go-review.googlesource.com/42250
Run-TryBot: Josh Bleecher Snyder <cdc248cc4d32e4fbfe04e45228163bcecd48a0b9@gmail.com>
Reviewed-by: Brad Fitzpatrick <ae9783c0b0efc69cd85ab025ddd17aa44cdc4aa5@golang.org>

golang/build,golang/build,golang/build,golang/build