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 <joao.fernandes@docker.com>

docs/reference/commandline/secret_create.md

@@ -36,7 +36,7 @@ For detailed information about using secrets, refer to [manage sensitive data wi
 ### Create a secret
 
 ```bash
-$ echo <secret> | docker secret create my_secret -
+$ printf <secret> | docker secret create my_secret -
 
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