welcome screen: fix prev tip for good

src/plugins/coreplugin/welcomemode.cpp

@@ -321,7 +321,7 @@ void WelcomeMode::slotNextTip()
 void WelcomeMode::slotPrevTip()
 {
     QStringList tips = tipsOfTheDay();
-    m_d->currentTip = tips.count()-((m_d->currentTip-1)%tips.count());
+    m_d->currentTip = ((m_d->currentTip-1)+tips.count())%tips.count();
     m_d->ui.didYouKnowTextBrowser->setText(tips.at(m_d->currentTip));
 }