Schlick Fresnel formula typo

[jeremyong]

Hi, first of all thanks for the awesome resource and write up - it's a great summary of the various ideas proposed over the years.

On the page discussing the Fresnel specular factor, the formula for the Schlick model is shown here:

It looks like the exponent got placed inside the parentheses on the right, and it should probably read:

$$F = F_0 + (F_{90} - F_0)\cdot (1 - u)^5$$