Update README.md

Make 30 -> :thirty (was :forty - not sure if that was a joke or an oversight!)

Author: cloojure

README.md

@@ -898,14 +898,14 @@ pairwise constructs as found in e.g. `let` and `cond`.
  (cond
  (= x 10) :ten
  (= x 20) :twenty
- (= x 30) :forty
+ (= x 30) :thirty
  :else :dunno)
 
  ;; much better
  (condp = x
  10 :ten
  20 :twenty
- 30 :forty
+ 30 :thirty
  :dunno)
  ```