Change update instructions to match ACP changes

PHPBB3-15028 is making wording changes to update instructions in ACP to eliminate the use of the wording 'automatic update'. 

The main phpBB.com website's navigation menu, in the Downloads dropdown also mentions,

<a href="/downloads/#update">Automatic Update packages for your phpBB release</a>

and the phpBB • Download phpBB 3.3 page has an 'Automatic Update' tab (phpBB • Download phpBB 3.3) where 'automatic updater' is used.