Fix docker exec command help messages

Signed-off-by: Lei Jitang <leijitang@huawei.com>
Upstream-commit: d8b17d785a03246cb3a081223a0242469af7d410
Component: engine

components/engine/docker/flags.go

@@ -62,7 +62,7 @@ func init() {
 			{"create", "Create a new container"},
 			{"diff", "Inspect changes on a container's filesystem"},
 			{"events", "Get real time events from the server"},
-			{"exec", "Run a command in an existing container"},
+			{"exec", "Run a command in a running container"},
 			{"export", "Stream the contents of a container as a tar archive"},
 			{"history", "Show the history of an image"},
 			{"images", "List images"},