For those wondering, this is what the resource at hand says about the matter:
"To be precise, when this mean is calculated, the sum of the squared deviations is divided by one less than the sample size rather than the sample size itself. There's no reason why it must be done this way, but this is the modern convention. It's not important that this seem the most natural measure of spread. It's the way it's done. You can just accept it (which I recommend) or you'll have to study the mathematics behind it. But, that's another course."
"To be precise, when this mean is calculated, the sum of the squared deviations is divided by one less than the sample size rather than the sample size itself. There's no reason why it must be done this way, but this is the modern convention. It's not important that this seem the most natural measure of spread. It's the way it's done. You can just accept it (which I recommend) or you'll have to study the mathematics behind it. But, that's another course."