image-guide: Use SLES15 instead of SLES12-SP3
image-guide: Use SLES15 instead of SLES12-SP3

Use current SUSE Linux Enterprise Version.
And fix typo.

Change-Id: I189cb4e1d8bfecc644dc66cca5fffa751b72d3bd

openstack/openstack-manuals,openstack/openstack-manuals,openstack/openstack-manuals,openstack/openstack-manuals