docs: fix yaml sections format at docs.docker.com

yaml sections in the documentation does not display well on
docs.docker.com. This is due to the syntax highlighting
which uses highlight.js and does not support yaml
currently.
The fix is to remove triple back ticks and indent instead.
We loose yaml syntax highlighting on github, but it displays
an acceptable version on both github and docs.docker.com.

Signed-off-by: Olivier Jacques <olivier.jacques@hp.com>

docs/notifications.md

@@ -36,17 +36,15 @@ order is not guaranteed.
 To setup a registry instance to send notifications to endpoints, one must add
 them to the configuration. A simple example follows:
 
-```yaml
-notifications:
-  endpoints:
-    - name: alistener
-	  url: https://mylistener.example.com/event
-      headers:
-        Authorization: [Bearer <your token, if needed>]
-      timeout: 500ms
-      threshold: 5
-      backoff: 1s
-```
+      notifications:
+        endpoints:
+          - name: alistener
+            url: https://mylistener.example.com/event
+            headers:
+              Authorization: [Bearer <your token, if needed>]
+            timeout: 500ms
+            threshold: 5
+            backoff: 1s
 
 The above would configure the registry with an endpoint to send events to
 `https://mylistener.example.com/event`, with the header "Authorization: Bearer