Book 1 Chapter 5.2: `hit_sphere` function -- fix discriminant test

[hollasch]

Discussed in #1057

^{Originally posted by LollipopFt August 20, 2022}
in the last line of hit_sphere, the code is:

return (discriminant > 0);

however, earlier in the same chapter the book mentioned

either positive (meaning two real solutions), negative (meaning no real solutions), or zero (meaning one real solution)

doesn't this mean the code should instead be

return (discriminant >= 0)

since discriminant = 0 means that the ray still touches the sphere and should thus be included?

[hollasch]

Fixed in #1124