Fixing domain and range of eventSchedule property

[thill-odi]

Related to issue #2277 - though there's a discrepancy between the error reported from Travis (type expected is EventSchedule) and the problem identified (domain is given as Schedule and range as Duration)

[RichardWallis]

Snippet for Site Improvements section of releases.html:

<li id="2277"><a href="https://github.com/schemaorg/schemaorg/issues/2277">Issue #2277</a>:
(implemented in <a href="https://github.com/schemaorg/schemaorg/pull/2388">PR #2388</a>):
Fixed error in definition of <a href="/eventSchedule">eventSchedule</a> property.
</li>

[ldodds]

Anything else to do for this to be merged @RichardWallis?