quad book2: fix u,v math notation

Author: hollasch

books/RayTracingTheNextWeek.html

@@ -2397,9 +2397,9 @@
 
 If $\mathbf{u}$ and $\mathbf{v}$ were guaranteed to be orthogonal to each other (forming a 90°
 angle between them), then this would be a simple matter of projecting $\mathbf{P}$ onto the two
-basis vectors $\mathbf{u}$ and $\mathbf{v}$. However, since we are not restricting $u$ and $v$ to be
-perpendicular, and allowing them to form any angle (other than zero), the math's a little bit
-trickier.
+basis vectors $\mathbf{u}$ and $\mathbf{v}$. However, since we are not restricting $\mathbf{u}$ and
+$\mathbf{v}$ to be perpendicular, and allowing them to form any angle (other than zero), the math's
+a little bit trickier.
 
   $$ \mathbf{P} = \mathbf{Q} + \alpha \mathbf{u} + \beta \mathbf{v}$$