Make if-not example clearer, fix the answer keys

I felt the example wasn't very clear as the conditional returned the
quoted form of `doom` when it evaluated to true or false. This way the
user needs to find a value that satisfies the if-not conditional so it
returns `doom` and not `more-doom`.

src/koans/07_conditionals.clj

@@ -33,9 +33,9 @@
                         :else __)))
 
   "Or your fate may be sealed"
-  (= __ (if-not (zero? __)
+  (= 'doom (if-not (zero? __)
           'doom
-          'doom))
+          'more-doom))
 
   "In case of emergency, sound the alarms"
   (= :sirens