[Book 3] Incorrect number of samples written for mixture density

[berkowitze]

[Minor]

Relatively sure the image here was generated with 10 samples, not 1000. Just was a bit confused because my image has a lot less noise.

raytracing.github.io/books/RayTracingTheRestOfYourLife.html

Line 3253 in dc8b773

Updating `main.cc` to 1000 samples per pixel (not listed) yields:

Book image:

My program with 1000 samples:

My program with 10 samples:

Happy to fix.