Adjust release process documentation on wiki

[sdarwin]

On the wiki page: https://github.com/boostorg/wiki/wiki/Releases%3A-Preparing

It mentions release-from-artifactory. Please modify the page to say release-from-fastly.

An earlier boost release, 1.86.0, used release-from-fastly which set the download links correctly.

With 1.87.0, there was likely a reversion to artifactory however that service is now disabled.

cc: @mclow @glenfe