[JENKINS-51660] Correct default URL proposed

core/src/main/resources/jenkins/install/SetupWizard/setupWizardConfigureInstance.jelly

@@ -106,7 +106,8 @@ THE SOFTWARE.
             $('root-url').focus();
             (function setInitialRootUrlFieldValue(){
               var iframeUrl = window.location.href;
-              var iframeRelativeUrl = '/setupWizard/setupWizardConfigureInstance';
+              // will let the trailing slash in the rootUrl 
+              var iframeRelativeUrl = 'setupWizard/setupWizardConfigureInstance';
               var rootUrl = iframeUrl.substr(0, iframeUrl.length - iframeRelativeUrl.length);
               // to keep only the root url
               var rootUrlField = $('root-url');