fix typo in functions lecture

Author: sglyon

lectures/python_fundamentals/functions.md

@@ -746,7 +746,7 @@ defined earlier.
 
 ```{hint}
 
-$\text{variance} = \sum_i (x_i - \text{mean}(x))^2$
+$\text{variance} = \frac{1}{N-1} \sum_i (x_i - \text{mean}(x))^2$
 
 ```
 
@@ -788,4 +788,4 @@ These can *only* be set by name.
 # Your code here.
 ```
 
-({ref}`back to text <dir2-4-4>`)
+({ref}`back to text <dir2-4-4>`)