Correct usage of the term "unit cube"

Resolves RayTracing#1555

Author: hollasch

CHANGELOG.md

@@ -10,6 +10,7 @@ Change Log / Ray Tracing in One Weekend
   - New    --
 
 ### In One Weekend
+  - Fix    -- Fixed usage of the term "unit cube" for a cube of diameter two (#1555)
 
 ### The Next Week
 

books/RayTracingInOneWeekend.html

@@ -2302,9 +2302,9 @@
 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 of the unit sphere. Pick
-a random point in the unit cube, where $x$, $y$, and $z$ all range from -1 to +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. 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.
 
   ![Figure [sphere-vec]: Two vectors were rejected before finding a good one
   ](../images/fig-1.11-sphere-vec.jpg)