remove public access modifiers on ref solution for nth prime

exercises/nth-prime/.meta/src/reference/java/PrimeCalculator.java

 import java.util.stream.IntStream;
 
-public final class PrimeCalculator {
-    public int nth(int nth) {
+final class PrimeCalculator {
+    int nth(int nth) {
         if (nth < 1) {
             throw new IllegalArgumentException();
         }