Use printf, not echo when creating secrets
Use printf, not echo when creating secrets

Update the docs so that users don't use `echo` when creating
secrets from STDIN. `echo` adds a trailing new line, so users
will probably be confused when their passwords don't work.

Signed-off-by: Joao Fernandes <80347794d2f9dea25484aa6e0caa7c9d20bee2ef@docker.com>

thaJeztah/cli,cpuguy83/cli,docker/cli,docker/cli,thaJeztah/cli,cpuguy83/cli