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>

docs/notifications.md

@@ -1,7 +1,7 @@
 ---
 description: Explains how to work with registry notifications
-keywords:
-- registry, on-prem, images, tags, repository, distribution, notifications, advanced
+keywords: registry, on-prem, images, tags, repository, distribution, notifications,
+  advanced
 menu:
   main:
     parent: smn_registry
@@ -347,4 +347,4 @@ provide acceptable guarantees, adding a transactional `Sink` to the registry
 is a possibility, although it may have an effect on request service time.
 Please see the
 [godoc](http://godoc.org/github.com/docker/distribution/notifications#Sink)
-for more information.
+for more information.