Fix mapping of `is_metadata_tag(true)` typedefs to annotation properties

[althonos]

OBO typedef frames are translated to annotation properties if they contain an is_metadata_tag: true clause. This is useful for creating typedefs in OBO that are used in property_value clauses.

However, in the semantic guide, the translation of relationship: clauses is wrong because it checks the is_class_level tag instead of the is_metadata_tag to translate a relationship to an AnnotationAssertion or an EquivalentClasses axioms. The is_class_level tag is only useful for translating an ObjectHasValue restriction.

[althonos]

ping @cmungall , I'm pretty sure this is correct and just a typo in the guide, but want to confirm before merging.

[cmungall]

Thanks!