QTranslator doc: use QCoreApplication::installTranslator()

The example snippet was using app.installTranslator() even
installTranslator() is a static function. And since app is a
QApplication instead a QCoreApplication this could lead to the
assumption that a QTranslator can only be used for gui applications.
Therefore use the correct static invocation
QCoreApplication::installTranslator().

Change-Id: Ia3ce00f25230c2fe2bdc159ec14c88c961924651
Reviewed-by: Paul Wicking <paul.wicking@qt.io>
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@qt.io>

src/corelib/doc/snippets/hellotrmain.cpp

@@ -56,13 +56,13 @@ int main(int argc, char *argv[])
     QTranslator translator;
     // look up e.g. :/translations/myapp_de.qm
     if (translator.load(QLocale(), QLatin1String("myapp"), QLatin1String("_"), QLatin1String(":/translations")))
-        app.installTranslator(&translator);
+        QCoreApplication::installTranslator(&translator);
 
     QPushButton hello(QCoreApplication::translate("main", "Hello world!"));
     hello.resize(100, 30);
 
     hello.show();
-    return app.exec();
+    return QCoreApplication::exec();
 }
 //! [0]