Fix docker exec help messages.

Signed-off-by: Lei Jitang <leijitang@huawei.com>

The docker exec command help messages "Run a command in an existing container"
should be "Run a command in a running container".
"an existing container" would confuse the users since docker exec exactly run a command 
in a running container.
docker-exec.1.md also description the docker exec command as 
"Run a process in a running container."