update JASP Compute Column instructions to JASP 0.18.3

[hugoquene]

In JASP 0.18.3, the procedure for adding a computed column has been modified. See revised notes on MS1 course, week 2, practical P2, handout for students (6 feb 2024). The updated instructions in that handout still need to be included into the textbook, in the section about JASP Compute Column (Ch.8??). This applies to the English and Dutch versions.