Use let scope to bing the agent

src/koans/21_agents.clj

@@ -1,8 +1,6 @@
-(def agent-example
-  "Agents provide shared access to mutable state."
-  (agent 10))
-
 (meditations
+ (let [agent-example (agent 10)]
+
    "Creating an Agent is as simple as assigning it a value"
    (= __ (instance? clojure.lang.Agent agent-example))
 
@@ -28,4 +26,7 @@
            (try
              (send agent-example + 1)
              (catch IllegalStateException e))
-          @agent-example)))
+           @agent-example))
+
+   "It's a good idea to clean up any not finished agents after you are done"
+   (shutdown-agents)))