base: Fix documentation of System.Environment.Blank
base: Fix documentation of System.Environment.Blank

This documentation was a bit unprofessional and the markup wasn't
correct.

Reviewers: hvr, alpmestan

Reviewed By: alpmestan

Subscribers: rwbarton, thomie, carter

Differential Revision: https://phabricator.haskell.org/D5026

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