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There are a few ways to look at this and it is up to you to determine the best course of action.
You have been given a lucky gift. You can choose not to say anything and you will keep those 5 points which may (but probably won't) make the difference in an A/A-, etc.
OTOH you also have an opportunity to make a very positive, honest impression on a professor who may someday make an important recommendation for you during the admissions process. Indeed, there is a very good chance that the professor WON'T lower your grade once you inform him of his mistake, in which case you have a win/win situation.
I would say that making an honest impression is more valuable than a small chance of losing 5 points that probably won't affect your overall grade anyway. This is based on my experiences with teachers. Yes I have been in your situation before and yes I said something about it and I felt like what I did was very beneficial later in the application process.
Final grades, otoh, I don't argue about either way. I've received an A before when I know that statistically I earned less than a 93, but it is up to the teacher to decide where to draw the line in the class and complaining about receiving a too high grade is disrespectful. A test grade, however is a separate matter.
So, you only tell the professor in order to make a good impression and get a good letter of rec. Interesting...