Change update instructions in ACP

Currently, the update instructions (whenever a new version is available for download) tells the administrator to use the automatic updater.
This was the recommend method for modified boards, but from 3.1 there should be no modifications in the core.

So, following a lot of support topic on phpbb.com and intl. support boards, the wording for language key UPDATE_INSTALLATION_EXPLAIN in language/en/install.php should be changed to respect the changed recommend method for updating a board