Makes the tab completion helper properly immutable

src/main/java/net/knarcraft/bookswithoutborders/utility/TabCompletionHelper.java

@@ -6,7 +6,11 @@ import java.util.List;
 /**
  * Helper class for getting string lists required for auto-completion
  */
-public class TabCompletionHelper {
+public final class TabCompletionHelper {
+
+    private TabCompletionHelper() {
+
+    }
 
     /**
      * Gets a list of booleans