Corrections to Upgrading Your ownCloud Server

Plz review and merge-- @PVince81 @MorrisJobke @LukasReschke, or any random passerby who likes to merge PRs.
I haven't tested this, but I would generally recommend running the script from inside `/var/www/owncloud` by cd-ing inside first.
Unless you say you tried it out and it worked, then it's fine.