Fix typo in eq-hashtable-ref-cell signature

[agspathis]

Currently:

> (let ([x (eq-hashtable-ref-cell (make-eq-hashtable) 'foo)])
    (list (boolean? x) (eq? x #t) (eq? x #f) (if x 'true 'false)))
(#t #f #f true)

[burgerrg]

Thanks again for finding and fixing this bug!