Include showStartupLaunchMessage in example ini file (#3452)

flameshot.example.ini

@@ -66,6 +66,9 @@
 ;; Launch at startup (bool)
 ;startupLaunch=true
 ;
+;; Show greeting message on startup (bool)
+;showStartupLaunchMessage=true
+;
 ;; Opacity of area outside selection (int in range 0-255)
 ;contrastOpacity=190
 ;