Small fix for CRAM Prolog

```lisp
(def-fact-group bla()
  (<- (robots are "super" cool)))
(prolog `(robots are "super" ?x))
```
gives ```NIL```, but should give ```cool``` in my opinion.
Should at least work for the current SBCL: https://github.com/sbcl/sbcl/blob/master/src/code/pred.lisp#L210