Just wanted to leave it here in case someone is following the course with the most recent version of Blender (albeit not recommended). The modulo math node is now split between truncated modulo and floored modulo.
While I don't know the difference (to be honest I don't even fully understand the basic modulo logic), if you pick truncated modulo it will work as expected.
Hi Nathalia,
I don't know exactly how the Modulo in Blender works either (I only know the mathematical Modulo, not the 'programmers Modulo'), but the Floored and Truncated have to do with the result of the Modulo and for positive values Floored and Truncated give the same result
Floored means rounding down to the closest Integer and Truncated means ignoring everything after the decimal point.
So the Floor of PI is 3 and PI truncated is also 3.
The Floor of -PI however is -4 while -PI truncated is -3.
(The mathematical Modulo is only defined on the Integers, so there is never any rounding.)