Doc: Fix snippet for QUrl::isValid() documentation

qDebug(...) expects a const char *, not a QString.

Change-Id: Ie4489c29440e328a732ed026eae3859eb8855ea5
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@qt.io>

src/corelib/doc/snippets/code/src_corelib_io_qurl.cpp

@@ -62,7 +62,7 @@ QUrl url = QUrl::fromEncoded("http://qt-project.org/List%20of%20holidays.xml");
 //! [2]
 bool checkUrl(const QUrl &url) {
     if (!url.isValid()) {
-        qDebug(QString("Invalid URL: %1").arg(url.toString()));
+        qDebug("Invalid URL: %s", qUtf8Printable(url.toString()));
         return false;
     }