docs: adding messing imports to the main files

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

@@ -1,2 +1,8 @@
 import java.io.*;
-import java.util.Arrays;
+import java.util.Arrays;
+
+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)));
+    }
+}