Correct default -A value in RTS flag usage info
Correct default -A value in RTS flag usage info

Reviewers: bgamari, erikd, simonmar

Reviewed By: bgamari

Subscribers: rwbarton, thomie, carter

Differential Revision: https://phabricator.haskell.org/D4441

sdiehl/ghc,sdiehl/ghc,sdiehl/ghc,sdiehl/ghc,sdiehl/ghc,sdiehl/ghc,sdiehl/ghc