[fix] prime sieve was missing the number 2

src/sieve.rs

@@ -7,6 +7,7 @@ pub fn get_primes(max_nr: usize) -> Vec<u64> {
     sieve[1] = false;
 
     let mut nr = 2;
+    primes.push(2);
 
     loop {
         check_off_multiples_of_nr(nr, &mut sieve);