Replace archive2 with archive

[grugel-maintro]

38 The messages aren't stored on the server as files. This is due to the use of archive2 instead of just archive as the prosody mam man page tells:

Legacy message archive

Early versions of mod_mam (which were available in prosody-modules) stored data in a store called 'archive2'. This store is now just called 'archive'.

mod_mam can be instructed to use the older store name if you still have data there.

archive_store = "archive2"; -- the old data storage = { archive2 = "sql"; }