Fix reference link error in Jenkins docs failure

This fix fixes one of the Jenkins docs failure:
https://jenkins.dockerproject.org/job/docs-docker-pr/9754/

There are 7 errors. This fix addresses one:
`* link error: (in page engine/userguide/storagedriver/device-mapper-driver.md) ../../reference/commandline/dockerd/#storage-driver-options`

Signed-off-by: Yong Tang <yong.tang.github@outlook.com>

docs/userguide/storagedriver/device-mapper-driver.md

@@ -658,4 +658,4 @@ data volumes.
 * [Select a storage driver](selectadriver.md)
 * [AUFS storage driver in practice](aufs-driver.md)
 * [Btrfs storage driver in practice](btrfs-driver.md)
-* [daemon reference](../../reference/commandline/dockerd/#storage-driver-options)
+* [daemon reference](../../reference/commandline/dockerd.md#storage-driver-options)