vhost: fix markdown formatting of docs heading

Signed-off-by: Alyssa Ross <[email protected]>

Author: alyssais

vhost/src/lib.rs

@@ -1,7 +1,7 @@
 // Copyright (C) 2019 Alibaba Cloud. All rights reserved.
 // SPDX-License-Identifier: Apache-2.0 or BSD-3-Clause
 
-//! Virtio Vhost Backend Drivers
+//! # Virtio Vhost Backend Drivers
 //!
 //! Virtio devices use virtqueues to transport data efficiently. The first generation of virtqueue
 //! is a set of three different single-producer, single-consumer ring structures designed to store