Fix Util-class declaration

Author: simonwep

README.md

@@ -369,6 +369,7 @@ Now you can access every files in the `test_statics` over the root adress `\`. I
 ```java
 FileProviderOptionsoptions = new FileProviderOptions();
 options.setExtensions("html", "css", "js"); // By default, all are allowed.
+
 /*
  * Activate the fallbacksearch.
  * E.g. if an request to <code>/js/code.js</code> was made but the

src/express/middleware/Middleware.java

@@ -7,7 +7,7 @@
  * @author Simon Reinisch
  * Class which provides middleware
  */
-public class Middleware {
+public final class Middleware {
 
   // Don't allow creating an instance from this class
   private Middleware() {

src/express/utils/Utils.java

@@ -15,7 +15,9 @@
 import java.util.Date;
 import java.util.TimeZone;
 
-public class Utils {
+public final class Utils {
+
+  private Utils() {}
 
   /**
    * Write all data from an InputStream in an String