Fix ICD formula for solved PDF in Importance Sampling (#1622)

[spayne]

1. This pull request fixes the ICD math. What was published was $8d^{\frac{1}{3}}$. What we want is: $2d^{\frac{1}{3}}$.
2. Note that when generating the estimate, the ICD gets cancelled out anyway. The code sum += x*x/pdf(x) is $\sum \frac{ic{d}^2}{pdf(icd)}=\sum \frac{ic{d}^2}{\frac{3}{8}ic{d}^2}=\frac{8}{3}$. The text doesn't mention this cancellation but does bring up the ICD again when talking about the PDF. I think it's wiser to stick to the PDF so I updated this reference as well.
3. I also updated CHANGELOG.md and the integrate_x_sq.cc

[spayne]

Requesting review from @RayTracing/reviewers