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/
Core views Dentist as subtype of MedicalOrganization and now LocalBus…
Removed mention of ProfessionalService as that type is being retired.
The health-lifesci extension also adds MedicalBusiness to this.
Noted fix for Dentist, #1283