[packaging] Tweak wording on postinst script
[packaging] Tweak wording on postinst script

Summary:
The copy here can give a little more guidance on what end-users
might want to do.

Closes #31

Test Plan: Ran it.

Reviewers: sgolemon, fredemmott, ptarjan

Reviewed By: ptarjan

Differential Revision: D1353165

autarch/packaging,hhvm/packaging,N3X15/hhvm-build,hhvm/packaging,N3X15/hhvm-build,hhvm/packaging,bauerj/packaging,newrelic-forks/packaging,mathieujobin/packaging