docs: adding messing imports to the main files

subjects/java/checkpoints/reverse-array/ExerciseRunner.java

@@ -1,3 +1,6 @@
+import java.io.*;
+import java.util.Arrays;
+
 public class ExerciseRunner {
     public static void main(String[] args) throws IOException {
         System.out.println(Arrays.toString(ReverseArray.reverse(new Integer[]{4, 2, 1, 3})));

subjects/java/checkpoints/rotate-array/ExerciseRunner.java

@@ -1,5 +1,2 @@
-public class ExerciseRunner {
-    public static void main(String[] args) throws IOException {
-        System.out.println(Arrays.toString(RotateArray.rotate(new Integer[]{4, 2, 1, 3}, 1)));
-    }
-}
+import java.io.*;
+import java.util.Arrays;

subjects/java/checkpoints/sort-array/ExerciseRunner.java

@@ -1,3 +1,6 @@
+import java.io.*;
+import java.util.Arrays;
+
 public class ExerciseRunner {
     public static void main(String[] args) throws IOException {
         System.out.println(Arrays.toString(SortArray.sort(new Integer[]{4, 2, 1, 3})));