Prefer non-bang async method, and sleep so the example succeeds

Without the sleep, you never see that it worked.

README.md

@@ -61,7 +61,7 @@ class Server
   end
 
   def run
-    loop { handle_message! @socket.read }
+    loop { async.handle_message @socket.read }
   end
 
   def handle_message(message)
@@ -95,8 +95,10 @@ addr = 'tcp://127.0.0.1:3435'
 server = Server.new(addr)
 client = Client.new(addr)
 
-server.run!
+server.async.run
 client.write('hi')
+
+sleep
 ```
 
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