correct the spec for stop_async/1 (#561)

Author: rciorba

doc/hackney.md

@@ -588,7 +588,7 @@ and headers of the response. and return
 ### stop_async/1 ###
 
 <pre><code>
-stop_async(Ref::<a href="#type-client_ref">client_ref()</a>) -&gt; ok | {error, req_not_found} | {error, term()}
+stop_async(Ref::<a href="#type-client_ref">client_ref()</a>) -&gt; {ok, <a href="#type-client_ref">client_ref()</a>} | {error, req_not_found} | {error, term()}
 </code></pre>
 <br />
 

src/hackney.erl

@@ -554,7 +554,7 @@ resume_stream(Ref) ->
                                                end).
 
 %% @doc stop to receive asynchronously.
--spec stop_async(client_ref()) -> ok | {error, req_not_found} | {error, term()}.
+-spec stop_async(client_ref()) -> {ok, client_ref()} | {error, req_not_found} | {error, term()}.
 stop_async(Ref) ->
   hackney_manager:stop_async_response(Ref).