Use spf13/cobra for docker diff

This fix is part of the effort to convert commands to spf13/cobra #23211.

Thif fix coverted command `docker diff` to use spf13/cobra

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

usage.go

@@ -12,7 +12,6 @@ var DockerCommandUsage = []Command{
 	{"build", "Build an image from a Dockerfile"},
 	{"commit", "Create a new image from a container's changes"},
 	{"cp", "Copy files/folders between a container and the local filesystem"},
-	{"diff", "Inspect changes on a container's filesystem"},
 	{"events", "Get real time events from the server"},
 	{"exec", "Run a command in a running container"},
 	{"history", "Show the history of an image"},