Bump cirros version to 0.6.3

This is the latest cirros release, featuring an updated kernel and some
fixes and added features, let's use it.

[0] https://github.com/cirros-dev/cirros/releases/tag/0.6.3

Change-Id: I2506fa713e0426789fa40a5f4f7fd4e963a158f0
diff --git a/doc/source/guides/nova.rst b/doc/source/guides/nova.rst
index 705d427..6b8aabf 100644
--- a/doc/source/guides/nova.rst
+++ b/doc/source/guides/nova.rst
@@ -122,7 +122,7 @@
 .. code-block:: shell
 
   $ openstack --os-compute-api-version 2.37 server create --flavor cirros256 \
-      --image cirros-0.6.2-x86_64-disk --nic none --wait test-server
+      --image cirros-0.6.3-x86_64-disk --nic none --wait test-server
 
 .. note:: ``--os-compute-api-version`` greater than or equal to 2.37 is
           required to use ``--nic=none``.