Fix server shutdown documentation

Add server.server_close() call to shutdown example to properly
release the port. Without this call, attempting to restart the
server on the same port results in "Address already in use" error.

Fixes #1068

Author: veeceey

docs/content/exporting/http/_index.md

@@ -24,6 +24,7 @@ to shutdown the server gracefully:
 ```python
 server, t = start_http_server(8000)
 server.shutdown()
+server.server_close()
 t.join()
 ```