Add dfn tag to doc-glossary example

[mattgarrish]

The definition for doc-glossary says you should identify the terms in the glossary and mentions using dl as an option, but ARIA 1.2 removes the association of dt and dd with the term and definition roles. dl, dt, and dd will eventually map to associationlist roles in ARIA 1.3. Only dfn identifies terms.

Since the associationlist roles are not specific to term/definition (and ARIA 1.2/ARIA in HTML currently have no associations for dl elements), I'm thinking we should add a dfn tag inside the dt in the example.