Use an explicit function argument for iterate

Also replace :hello with "hello" to avoid potential confusion since
:hello is also a function.

refs #75

resources/koans.clj

@@ -127,7 +127,7 @@
                           [0 1 2 3 4]
                           10
                           95
-                          (range 20)
+                          [1 2 4 8 16 32 64 128]
                           :a]
                     "___" [(fn [x] x)]}]