digitalAssetContract: Update TODOs, XML example

Added .s
I think I figured it out. There is an element inside `<digitalAssetContract>` called issue: https://github.com/Open-Transactions/opentxs/blob/be111238c0feb569462b2e710e7570c00aa3d8db/src/core/AssetContract.cpp#L795.

We need to add this to the description

* Element `issue`. TODO
  * Attribute `company`.
  * Attribute `email`.
  * Attribute `contractURL`.
  * Attribute `type`. (This is the one you meant)