Updating sllgen.rkt

As per bug request #13831 (http://bugs.racket-lang.org/query/?debug=&database=default&cmd=view+audit-trail&cmd=view&pr=13831)

Since Racket v. 5.3.4, support for SLLGEN in EOPL package.

This simple fix, re-enables the feature which is clearly listed in the documentation.
http://docs.racket-lang.org/eopl/index.html?q=sllgen#%28def._%28%28lib._eopl%2Feopl..rkt%29._sllgen~3amake-rep-loop%29%29