docs: clarify that node promote/demote must be executed from a manager

Fixed wording per https://github.com/docker/docker.github.io/issues/7281

Signed-off-by: Anne Henmi <anne.henmi@docker.com>

Update node_promote.md

docs/reference/commandline/node_promote.md

@@ -26,8 +26,7 @@ Options:
 
 ## Description
 
-Promotes a node to manager. This command targets a docker engine that is a
-manager in the swarm.
+Promotes a node to manager. This command can only be executed on a manager node.
 
 ## Examples