[Module 5, Lesson 4] Mismatch between mathematical equation and implementation

[mesnardo]

Notebook: Lesson 4 of Module 5.

Problem: In the section "More difficult Poisson problems", there is a mismatch in the last of the source term between the mathematical equation (with cos) and the Python implementation (with sin).

Solution: We need to fix the last term of the Python implementation (sin -> cos).