Fix the reno format of image-clients-as-library

As reviews of Iab90412ff4e793b0bb395b8ba8b6d35cef316dec , the reno
format seems a bit weird. So this patch makes these items bullet
list.

Change-Id: I0b4b7c50f957fa862ebcd4ad833aab1179897961
diff --git a/releasenotes/notes/image-clients-as-library-86d17caa26ce3961.yaml b/releasenotes/notes/image-clients-as-library-86d17caa26ce3961.yaml
index 630f8ed..b50ed38 100644
--- a/releasenotes/notes/image-clients-as-library-86d17caa26ce3961.yaml
+++ b/releasenotes/notes/image-clients-as-library-86d17caa26ce3961.yaml
@@ -1,10 +1,12 @@
 ---
 features:
-  - Define image service clients as libraries
+  - |
+    Define image service clients as libraries
     The following image service clients are defined as library interface,
     so the other projects can use these modules as stable libraries
     without any maintenance changes.
-      **image_members_client**
-      **namespaces_client**
-      **resource_types_client**
-      **schemas_client**
+
+      * image_members_client
+      * namespaces_client
+      * resource_types_client
+      * schemas_client