Update `type` in Identifier Scheme documentation

This better reflects what seems to be the currently accepted identifier format (and what the validator actually checks for), except we do not yet have examples of non-ascii letters in division types.

I realize setting policy based on current validator implementation seems backwards, but codifying common practice is acceptable, right?
Merge pull request #53 from jmwaura/patch-2

Update `type` in Identifier Scheme documentation