Update agent.markdown (elixir-lang#767)

Looks like two different explanations of step were left, I left the one I felt was a better descriptor of what was going on.

One thing I wasn't sure about was that even though we are calling Agent.stop with only one argument, the function itself has an arity of three so I thought it would be better to have that represented over Agent.stop/1

Author: nfpiche

getting-started/mix-otp/agent.markdown

@@ -48,7 +48,7 @@ iex> Agent.stop(agent)
 :ok
 ```
 
-We started an agent with an initial state of an empty list. We updated the agent's state, adding our new item to the head of the list. The second argument of [`Agent.update/3`](/docs/stable/elixir/Agent.html#update/3) is a function that takes the agent's current state as input and returns its desired new state. Finally, we retrieved the whole list. The second argument of [`Agent.get/3`](/docs/stable/elixir/Agent.html#get/3) is a function that takes the state as input and returns the value that `Agent.get/3` itself will return. Once we are done with the agent, we can call `Agent.stop/1` to terminate the agent process.
+We started an agent with an initial state of an empty list. We updated the agent's state, adding our new item to the head of the list. The second argument of [`Agent.update/3`](/docs/stable/elixir/Agent.html#update/3) is a function that takes the agent's current state as input and returns its desired new state. Finally, we retrieved the whole list. The second argument of [`Agent.get/3`](/docs/stable/elixir/Agent.html#get/3) is a function that takes the state as input and returns the value that [`Agent.get/3`](/docs/stable/elixir/Agent.html#get/3) itself will return. Once we are done with the agent, we can call [`Agent.stop/3`](/docs/stable/elixir/Agent.html#stop/3) to terminate the agent process.
 
 Let's implement our `KV.Bucket` using agents. But before starting the implementation, let's first write some tests. Create a file at `test/kv/bucket_test.exs` (remember the `.exs` extension) with the following: