Fix FFFFFFFUUUUUUUUUUU example

technically the previous version doesn't produce the desired string, even with infinite memory. replace (repeat "U") with (repeat 20 "U") to see why:

user[132]: (apply str (repeat 7 "F") (repeat 20 "U"))
user[132]= "clojure.lang.LazySeq@5a43eae5UUUUUUUUUUUUUUUUUUUU"

user[133]: (apply str (concat (repeat 7 "F") (repeat 20 "U")))
user[133]= "FFFFFFFUUUUUUUUUUUUUUUUUUUU"