fix more frontmatter keywords values (#439)

* fix format of frontmatter keyword entry in some .md files

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

engine/security/https.md

@@ -3,8 +3,7 @@ aliases:
 - /engine/articles/https/
 - /articles/https/
 description: How to setup and run Docker with HTTPS
-keywords:
-- docker, docs, article, example, https, daemon, tls, ca,  certificate
+keywords: docker, docs, article, example, https, daemon, tls, ca,  certificate
 menu:
   main:
     parent: smn_secure_docker
@@ -215,4 +214,4 @@ flags:
 ## Related information
 
 * [Using certificates for repository client verification](certificates.md)
-* [Use trusted images](trust/index.md)
+* [Use trusted images](trust/index.md)