The ProfessionalService type has been retired. Replace it's mention for Dentist (a core term) with LocalBusiness.

Note that the health-lifesci vocab also adds MedicalBusiness, but that's a separate issue (as is "Dentistry" vs "Dentist").

This is quick fix addressing a point brought up in review of the 3.1 release candidate #1212, see https://lists.w3.org/Archives/Public/public-schemaorg/2016Aug/