Update "usage" documentation, affiliations.

[Raphael-Gazzotti]

The affiliation attribute in omcore.Person should be an array of embedded objects of type Affiliation, not a single object.

Current usage:

stan = omcore.Person(
    given_name="Stan",
    family_name="Laurel",
    affiliations=omcore.Affiliation(member_of=mgm, start_date=date(1942, 1, 1))
)

Corrected usage in the documentation:

stan = omcore.Person(
    given_name="Stan",
    family_name="Laurel",
    affiliations=[omcore.Affiliation(member_of=mgm, start_date=date(1942, 1, 1))]
)