Allow Human Readable Identifiers (HRID) to have rm_closures starting by numbers

In the Archetype Identification Specification (http://www.openehr.org/releases/AM/latest/docs/Identification/Identification.html#_human_readable_identifier_hrid) the rm_closure part grammar specifies that they need to start with a letter. The example itself (ISO-ISO13606-ENTRY) highlights this problem, as ISO-13606-ENTRY should be good enough.