envsetup: Clarify lunch help for listing targets

It is easy to misinterpret the use of list_products and other commands
as lunch's subcommand.

Bug: None
Test: lunch --help
Change-Id: Idf9a9d2d9977191ca5dd16a1e4994fe1c3a65c12
Signed-off-by: Sachin Parekh <sachinparekh@google.com>