fix small documentation error

Change a change ID to a bug ID to make the cinder documentation correct.

Change-Id: I8dc69368b432de8e8bb7669988566acffe619a45
Closes-Bug: #1603818

doc/cinder-guide.rst

@@ -25,7 +25,7 @@ a real physical volume or a loopback mounted file for development.
   present is via a Ceph deployment.  If community members disagree
   with this decision, please report the specific use case in the
   Cinder bug tracker here:
-    `_bug 306756  <https://launchpad.net/bugs/306756>`__.
+    `_bug 1571211  <https://launchpad.net/bugs/1571211>`__.
 
 
 Create a Volume Group