In the phrase "doctor appointment" or "doctors appointment" the word doctor serves as a descriptive adjective that describes what kind of an appointment it is. In such a phrase the emphasis is on the word appointment. In the phrase "doctor's appointment" or "doctors' appointment" the possessive form of doctor shifts the emphasis away from appointment. Both ways are correct. It just depends on how you want your sentence to read. My preference is to use the possessive apostrophe only when it is important to express ownership.