Standardise the double dashes to em dashes in TCW20

[trishaoconnor]

In keeping with the TEI Guidelines issue #2155, the double dashes in TCW 20 should be standardised to em dashes for consistency across the TEI repos.

This document is intended to set out the way things are currently managed in the editing of the TEI Guidelines. General notes on the rationale for this state -- why it is the way it is -- may be added here later.