Update the outdated link in user-guide
Update the outdated link in user-guide

The link to `slides` in user-guide.rst gives 404. So this patch
updates the link.

Change-Id: I0ebf7c5ffe415b0c4325fef5ecade58773f6eeb6
Closes-bug: #1651065

SauloAislan/ironic,openstack/ironic,NaohiroTamura/ironic,NaohiroTamura/ironic,pshchelo/ironic,SauloAislan/ironic,pshchelo/ironic,openstack/ironic