[OTSpec] Erratum, new OpenType Layout language system tag

A new erratum has been posted for OpenType 1.8.1: There was an error in describing numeric values for some operator codes for the CFF2 table. See the errata page for OT 1.8.1 for details.

Also, a new language system tag has been registered for Chinantec languages. Chinantec is a family of languages spoken in South/Southwest Mexico. It's reported that there are some typographic distinctions needed for Chinantec languages, though not between them. So, a language system tag was needed for the group of languages. See the Language system tags page for details.