Fix text around unit sphere random pt generation

Resolves #1603

Author: hollasch

CHANGELOG.md

@@ -12,7 +12,7 @@ Change Log / Ray Tracing in One Weekend
               height for code listings in both print and browser.
 
 ### In One Weekend
-  - Fix    -- Fixed usage of the term "unit cube" for a cube of diameter two (#1555)
+  - Fix    -- Fixed usage of the term "unit cube" for a cube of diameter two (#1555, #1603)
   - Fix    -- Fixed broken highlighting on some code listings (#1600)
 
 ### The Next Week

books/RayTracingInOneWeekend.html

@@ -2302,9 +2302,10 @@
 3. Invert the normalized vector if it falls onto the wrong hemisphere
 
 <div class='together'>
-First, we will use a rejection method to generate the random vector inside the unit sphere. Pick a
-random point in the cube enclosing the unit sphere (that is, where $x$, $y$, and $z$ are all in the
-range $[0,1]$), and reject this point if it is outside the unit sphere.
+First, we will use a rejection method to generate the random vector inside the unit sphere (that is,
+a sphere of radius 1). Pick a random point inside the cube enclosing the unit sphere (that is, where
+$x$, $y$, and $z$ are all in the range $[-1,+1]$). If this point lies outside (or on) the unit
+sphere, then generate a new one until we find one that lies inside the unit sphere.
 
   ![Figure [sphere-vec]: Two vectors were rejected before finding a good one
   ](../images/fig-1.11-sphere-vec.jpg)