fix type of keywords entry in frontmatter (in /registry/) (#517)

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

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

docs/index.md

@@ -1,9 +1,8 @@
 ---
+description: High-level overview of the Registry
+keywords: registry, on-prem, images, tags, repository, distribution
 redirect_from:
 - /registry/overview/
-description: High-level overview of the Registry
-keywords:
-- registry, on-prem, images, tags, repository, distribution
 title: Docker Registry
 ---
 
@@ -67,4 +66,4 @@ Now stop your registry and remove all data
 
 You should now read the [detailed introduction about the
 registry](introduction.md), or jump directly to [deployment
-instructions](deploying.md).
+instructions](deploying.md).