Broken link in web bonus problem week 4 #6

[Emily Deibert]

The link in this problem leads to a 404 error missing page.

http://www.math.toronto.edu/courses/apm346h1/20159/PDE-textbook/Chapter2/S2.7.P.html#problem-2.7.P.5

[Victor Ivrii]

Fixed (but force reload may be required)