<build/devenv> fix wording on --v2cart usage output

This option to "build/devenv launch" had inaccurate documentation in
the usage output (build/devenv help launch). The option was also
undocumented in README.md. This commit addresses both issues.
Merge pull request #56 from dobbymoodge/devenv-v2cart-wording

Merged by openshift-bot
prune sqe instances