Add as valid attribute combination with only start in ContentKeyPeriod

[sanao]

Following issue #101 and the list of valid attribute combinations for the ContentKeyPeriod tag (see https://dashif.org/docs/CPIX2.4/Cpix.html#schema-datamodel-contentkeyperiod), I believe explicitly stating the combination with only the start attribute will help clarify that the end attribute is optional.

In this case, the period remains "open" until a new period is created to follow it.