add example for configureDevServerOptions

Author: Kocal

index.js

@@ -660,7 +660,10 @@ class Encore {
      * ```
      * Encore.configureDevServerOptions(function(options) {
      *     // change the configuration
-     *     options
+     *     options.https = {
+     *         key: '<your SSL cert key content>',
+     *         cert: '<your SSL cert content>',
+     *     };
      * });
      * ```
      *