Correct optimization flags documentation
In a previous change (commit 4fd6207ec6960c429e6a1bcbe0282f625010f52a),
the users guide was moved from XML to the RST format. This process
introduced a typo: "No -O*-type option specified:" was changed to "-O*"
(which is not correct). This change fixes it.
See result in: https://prnt.sc/fh332n
Fixes ticket Trac #13756.
Reviewers: austin, bgamari
Reviewed By: bgamari
Subscribers: rwbarton, thomie
GHC Trac Issues: Trac #13756
Differential Revision: https://phabricator.haskell.org/D3631