Fix sendfile.py example

[python273]

because of @validator decorator, the fn returns wsgiref.validate.IteratorWrapper instead of gunicorn.http.wsgi.FileWrapper which breaks sendfile

gunicorn/gunicorn/workers/sync.py

Lines 181 to 182 in 69c508a

	if isinstance(respiter, environ['wsgi.file_wrapper']):
	resp.write_file(respiter)

[benoitc]

thanks!