Revert "Change the User Guide URL to SourceForge."

This reverts commit 47c0fd15f9.

widgets/mainwindow.cpp

@@ -2655,7 +2655,9 @@ void MainWindow::on_actionQSG_X250_M3_triggered()
 
 void MainWindow::on_actionOnline_User_Guide_triggered()      //Display manual
 {
-  QDesktopServices::openUrl (QUrl {"https://wsjt.sourceforge.io/wsjtx-main_en.html"});
+#if defined (CMAKE_BUILD)
+  m_manual.display_html_url (QUrl {PROJECT_MANUAL_DIRECTORY_URL}, PROJECT_MANUAL);
+#endif
 }
 
 //Display local copy of manual