Reword paragraph for overloaded term "object"

Resolves #384

Author: hollasch

books/RayTracingInOneWeekend.html

@@ -1007,9 +1007,9 @@
 
 </div>
 
-We'll use shared pointers in our code, because it allows multiple objects to use a common object
-(for example, a bunch of spheres that all use the same material), and because it makes memory
-management automatic and easier to reason about.
+We'll use shared pointers in our code, because it allows multiple geometries to share a common
+instance (for example, a bunch of spheres that all use the same texture map material), and because
+it makes memory management automatic and easier to reason about.
 
 `std::shared_ptr` is included with the `<memory>` header.