Update reference resolution (#2726)

As in the design.md file we are informing that it is more performing using string concatenation seems appropiate to use them instead of the String.format method

exercises/concept/log-levels/.meta/src/reference/java/LogLevels.java

@@ -8,6 +8,6 @@ public static String logLevel(String logLine) {
     }
 
     public static String reformat(String logLine) {
-        return String.format("%s (%s)", message(logLine), logLevel(logLine));
+        return message(logLine) + " (" + logLevel(logLine) + ")";
     }
 }