Changed reference from :timer.sleep/1 to Process.sleep/1 for 1.3 (elixir-lang#786)

Author: uri

getting-started/mix-otp/agent.markdown

@@ -174,9 +174,9 @@ This distinction is important. If there are expensive actions to be done, you mu
 
 ```elixir
 def delete(bucket, key) do
-  :timer.sleep(1000) # puts client to sleep
+  Process.sleep(1000) # puts client to sleep
   Agent.get_and_update(bucket, fn dict ->
-    :timer.sleep(1000) # puts server to sleep
+    Process.sleep(1000) # puts server to sleep
     Map.pop(dict, key)
   end)
 end