"I go to the doctor."
Translation:Ich gehe zum Arzt.
If youre visiting a doctors office (to get a medical check) then you just say "zum Arzt". Its not about the sex of the doctor, its a general statement that youre going to a person of the profession doctor (and here the masculinum word is used in nearly all cases).
Ich gehe zum Bäcker/zum Metzger would be the same case. The sex of the person whos selling or creating the food does not matter for this.
Now you could say it depends on the situation, if this sentence is in a situation where youre talking about a specific person, then it might be Ärztin in german.
It's not incorrect - just a missing alternative. Report it if you'd like.
This sentence is from the Pearson cooperation; I'm not sure whether they look at the reports.
That said, "I go to the doctor" can mean two things: you might be thinking of a specific doctor, or you might just be saying that you want to use the services of a doctor. (A bit like how "I read the newspaper" and "He is playing the piano" can refer to a specific newspaper or piano or they might just refer to the general idea of newspapers and pianos.)
In the first case, Ich gehe zum Arzt / zur Ärztin might be used depending on the gender of that particular doctor; in the second, I would use Ich gehe zum Arzt even if the doctor I visit will be female -- this is a sort of abstract use.