9862 fix typo in comment in vdev_impl.h

Reviewed by: Matthew Ahrens <mahrens@delphix.com>
Reviewed by: Brian Behlendorf <behlendorf1@llnl.gov>
Approved by: Robert Mustacchi <rm@joyent.com>

usr/src/uts/common/fs/zfs/sys/vdev_impl.h

@@ -297,7 +297,7 @@ struct vdev {
 
 	/*
 	 * The queue depth parameters determine how many async writes are
-	 * still pending (i.e. allocated by net yet issued to disk) per
+	 * still pending (i.e. allocated but not yet issued to disk) per
 	 * top-level (vdev_async_write_queue_depth) and the maximum allowed
 	 * (vdev_max_async_write_queue_depth). These values only apply to
 	 * top-level vdevs.