Add py3 compat to example in docstring of wsgi mod

PR #78 by @jaymcgrath

cheroot/wsgi.py

@@ -8,7 +8,7 @@ def my_crazy_app(environ, start_response):
         status = '200 OK'
         response_headers = [('Content-type','text/plain')]
         start_response(status, response_headers)
-        return ['Hello world!']
+        return [b'Hello world!']
 
     addr = '0.0.0.0', 8070
     server = wsgi.Server(addr, my_crazy_app)