docs(java piscine): Add missing imports to CatInFile exercise runner (#2142)

subjects/java/piscine/CatInFile/README.md

@@ -28,6 +28,8 @@ Here is a possible ExerciseRunner.java to test your function :
 
 ```java
 import java.io.*;
+import java.nio.file.Files;
+import java.nio.file.Paths;
 
 public class ExerciseRunner {
     public static void main(String[] args) throws IOException {