fig.rand-vector: update for clarity

Figure [rand-vector], In One Weekend. Also update preceding text.

Fixes #142

Author: Peter Shirley

books/RayTracingInOneWeekend.html

@@ -1363,12 +1363,13 @@
 the chapter.)
 
 <div class='together'>
-There are two unit radius spheres tangent to the hit point $p$ of a surface. These two spheres
-have a center of $(p + N)$ and $(p - N)$, where $N$ is the normal of the surface. The sphere with a
-center at $(p - N)$ is considered _inside_ the surface, whereas the sphere with center $(p + N)$ is
-considered _outside_ the surface. Select the tangent unit radius sphere that is on the same side of
-the surface as the ray origin. Pick a random point $s$ inside this unit radius sphere and send a
-ray from the hit point $p$ to the random point $s$:
+There are two unit radius spheres tangent to the hit point $p$ of a surface. These two spheres have
+a center of $(p + \vec{N})$ and $(p - \vec{N})$, where $\vec{N}$ is the normal of the surface. The
+sphere with a center at $(p - \vec{N})$ is considered _inside_ the surface, whereas the sphere with
+center $(p + \vec{N})$ is considered _outside_ the surface. Select the tangent unit radius sphere
+that is on the same side of the surface as the ray origin. Pick a random point $s$ inside this unit
+radius sphere and send a ray from the hit point $p$ to the random point $s$ (this is the vector
+$(s-p)$):
 
   ![Figure [rand-vector]: Generating a random diffuse bounce ray](../images/fig.rand-vector.jpg)