fix type of keywords entry in frontmatter (in /engine/) (#552)

keywords should be a string, not an array containing one string

Signed-off-by: Gaetan de Villele <gdevillele@gmail.com>

engine/security/https.md

@@ -1,10 +1,9 @@
 ---
+description: How to setup and run Docker with HTTPS
+keywords: docker, docs, article, example, https, daemon, tls, ca,  certificate
 redirect_from:
 - /engine/articles/https/
 - /articles/https/
-description: How to setup and run Docker with HTTPS
-keywords:
-- docker, docs, article, example, https, daemon, tls, ca,  certificate
 title: Protect the Docker daemon socket
 ---
 
@@ -210,4 +209,4 @@ flags:
 ## Related information
 
 * [Using certificates for repository client verification](certificates.md)
-* [Use trusted images](trust/index.md)
+* [Use trusted images](trust/index.md)