use rec( ("$1") := 2 ) instead of rec( $1 := 2 )

as the latter does not work anymore

From: Alexander Konovalov <alexk@mcs.st-andrews.ac.uk>
Subject: $ in identifiers
Date: 4. Februar 2014 23:37:32 MEZ
To: Mohamed Barakat <mohamed.barakat@rwth-aachen.de>

Dear Mohamed,

sorry for not reporting this before: actually, dollar is no longer allowed in
identifiers. Would it be possible to fix this in GradedRingForHomalg?