typo error in section 2.6 Rule Class

[riannella]

In the section https://www.w3.org/TR/odrl-model/#rule

The sentence
Explicit sub-properties of the abstract relation, relation and failure properties must be used, the choice depending on the subclass of Rule in question.

Should have been
Explicit sub-properties of the abstract relation, function and failure properties must be used, the choice depending on the subclass of Rule in question.

(submitted by @ksridhar)

[riannella]

Note: this has already been reported and added as errata:
#301