Fix error in dockerd.md for incorrect cluster-store-opts example.

This fix fixes an error in documentation (dockerd.md). In the
example given by dockerd.md, the option `cluster-store-opts`
is assigned with an array but this option can only be assigned
as a map.

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

docs/reference/commandline/dockerd.md

@@ -913,7 +913,7 @@ This is a full example of the allowed configuration options in the file:
 	"pidfile": "",
 	"graph": "",
 	"cluster-store": "",
-	"cluster-store-opts": [],
+	"cluster-store-opts": {},
 	"cluster-advertise": "",
 	"max-concurrent-downloads": 3,
 	"max-concurrent-uploads": 5,