docs: fixing typos

subjects/java/checkpoints/system-logger/README.md

@@ -21,7 +21,7 @@ Here is a possible `ExerciseRunner.java` to test your function :
 ```java
 public class ExerciseRunner {
     public static void main(String[] args) {
-        System.out.println(SystemLog.systemLog('message'));
+        System.out.println(SystemLog.systemLog("message"));
     }
 }
 ```

subjects/java/checkpoints/universal-greeting/README.md

@@ -27,7 +27,7 @@ Here is a possible `ExerciseRunner.java` to test your function :
 ```java
 public class ExerciseRunner {
     public static void main(String[] args) {
-        System.out.println(UniversalGreeting.greeting('EN'));
+        System.out.println(UniversalGreeting.greeting("EN"));
     }
 }
 ```