Documentation error in base tables link_classes

In /horizon/tables/base.py there is a mistake in the
documentation here:
https://github.com/openstack/horizon/blob/master/horizon/tables/base.py#L197

The name of the attribute is "link_classes", not
"classes" like the example suggests. That could be confusing for developers.

Change-Id: I61c1807f61364a152a5a51fcec8e9158dfa7067c
Closes-Bug: #1392721
Merge "Documentation error in base tables link_classes"