[docs] Wrong return type for Property#getFirstValue

[nicolo-ribaudo]

getFirstValue is documented as returning a string at https://kewisch.github.io/ical.js/api/ICAL.Property.html#getFirstValue, but sometimes it returns an object:

ical.js/test/property_test.js

Lines 483 to 486 in 3553349

	let value = subject.getFirstValue();
	assert.equal(subject.type, 'date-time');
	assert.instanceOf(value, ICAL.Time);