Update the description of project in releasenotes.

Change-Id: I1fe0e52d2e049d34666a6d94bac1d85b91a78b19

Author: qiangtang

releasenotes/source/conf.py

@@ -285,7 +285,7 @@
     u'OpenStackclient Release Notes Documentation',
     u'OpenStackclient Developers',
     'OpenStackClientReleaseNotes',
-    'One line description of project.',
+    'A unified command-line client for OpenStack.',
     'Miscellaneous',
 )]