Merge pull request #400 from RayTracing/fix-399

Fix OCR error in code, where "= O" should be "= 0"

Author: hollasch

books/RayTracingTheNextWeek.html

@@ -880,7 +880,7 @@
 
     class texture {
         public:
-            virtual vec3 value(double u, double v, const vec3& p) const = O;
+            virtual vec3 value(double u, double v, const vec3& p) const = 0;
     };
 
     class constant_texture : public texture {