Fix server websocket example

Author: asvetlov

docs/web.rst

@@ -396,7 +396,7 @@ using response's methods:
         ws = web.WebSocketResponse()
         await ws.prepare(request)
 
-        while True:
+        while not ws.closed:
             msg = await ws.receive()
 
             if msg.tp == aiohttp.MsgType.text:
@@ -407,7 +407,7 @@ using response's methods:
             elif msg.tp == aiohttp.MsgType.close:
                 print('websocket connection closed')
             elif msg.tp == aiohttp.MsgType.error:
-                print('ws connection closed with exception %s',
+                print('ws connection closed with exception %s' %
                       ws.exception())
 
         return ws