Fix little typo at _list issues_ docs

** (Protocol.UndefinedError) protocol String.Chars not implemented for %Tentacat.Client{auth: %{access_token: "xxxxx"}, endpoint: "https://api.github.com/"} of type Tentacat.Client (a struct). This protocol is implemented for the following type(s): Float, DateTime, Time, List, Version.Requirement, Atom, Integer, Version, Date, BitString, NaiveDateTime, URI

lib/tentacat/issues/issues.ex

@@ -8,7 +8,7 @@ defmodule Tentacat.Issues do
   ## Example
 
       Tentacat.Issues.list "elixir-lang", "elixir"
-      Tentacat.Issues.list "elixir-lang", "elixir", client
+      Tentacat.Issues.list client, "elixir-lang", "elixir"
 
   More info at: https://developer.github.com/v3/issues/#list-issues
   """