Promote a Node to Manager

[githubfromgui]

In the sentence "This command targets a docker engine that is a manager in the swarm. " it should not be "This command targets a docker engine that is NOT a manager in the swarm." ?

Because we want to promote a NOT manager to BE a manager.

File: edge/engine/reference/commandline/node_promote.md

Still valid in current version of the docs.

[ahh-docker]

PR is here: docker/cli#1578 Being handled by @andrewhsu's team. Fix done.