It's not a fake, but you did not have to click on it. Avast is reminding people that the 4.8 support is being discontinued shortly after the end of this year and encouraging people to upgrade to FREE (or if you want a different product like Pro or AIS) the newest version, which is 5.0.677.
Do you want to continue with the Free version and upgrade to the newer version? Or did you already start the install for the paid version?