Correct -g flag description

Authored by niteria on Mar 5 2018, 1:10 PM.

Description

Correct -g flag description

Since 7aaeaf81ea95c36fe1dc4da449cf6092a792fd09
-g1 and -g2 are actually different.
The tutorial below is correct.

Test Plan: harbormaster

Reviewers: bgamari, simonmar

Reviewed By: bgamari

Subscribers: rwbarton, thomie, carter

Differential Revision: https://phabricator.haskell.org/D4471