face -> faces and small english

Author: trevordblack

books/RayTracingTheNextWeek.html

@@ -1911,9 +1911,9 @@
 This is very noisy because the light is small. But we have a problem: some of the walls are facing
 the wrong way. We haven't specified that a diffuse material should behave differently on different
 faces of the object, but what if the Cornell box had a different pattern on the inside and outside
-walls? The rectangle objects are described such that their front face is always in the $(1, 0, 0)$,
-$(0, 1, 0)$, or $(0, 0, 1)$ directions. We need a way to switch the faces of an object. Let’s make
-a hittable that does nothing but hold another hittable, but flips the face:
+walls? The rectangle objects are described such that their front faces are always in the
+$(1, 0, 0)$, $(0, 1, 0)$, or $(0, 0, 1)$ directions. We need a way to switch the faces of an
+object. Let’s make a hittable that does nothing but hold another hittable, and flips the face:
 
     ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ C++
     class flip_face : public hittable {