Ethos refers to an argument that's based on the credibility of the person that's making the argument.
Ethos simply means establishing the authority to speak on a particular object subject. It's the persuasion and the argument whereby the knowledge, good moral character and the credibility of the speaker is shown.
In this case, the example given isn't an example of ethos because dentists in this case is a broad term and therefore does not give credibility from a particular dentist or probably a dental research.
