Shouldn't this be "Qelpu' maH." as doctors is plural? Or is it like the term "Klingon" in that "tlhIngan" can sometimes be the plural?
In almost all cases, the plural suffix is optional. Qel can refer to one doctor or multiple doctors; which it is is determined by context or other words in the sentence. In this case, it's multiple doctors because it's maH we, us.