Typo in 02-SystemThreads/Rayon.md

[weitecklee]

In 02-SystemThreads/Rayon.md, in the code example for finding prime numbers, the elapsed time is calculated immediately after creating the vector of numbers, not after finding and sorting the primes.

// Print primes below 1,000
let now = Instant::now();
let numbers: Vec<u64> = (2 .. 1_000_000).collect();
let elapsed = now.elapsed();
let mut primes: Vec<&u64> = numbers.par_iter().filter(|&n| is_prime(*n as u32)).collect();
primes.sort();
println!("{primes:?}");
println!("It took {} ms to find {} primes", elapsed.as_millis(), primes.len());