Replace archive2 with archive

#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](https://prosody.im/community_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";
> }

conf.d/02-storage.cfg.lua

@@ -14,7 +14,7 @@ archive_store = "archive2" -- Use the same data store as prosody-modules mod_mam
 
 storage = {
   -- this makes mod_mam use the sql storage backend
-  archive2 = os.getenv("STORAGE");
+  archive = os.getenv("STORAGE");
 }
 
 -- https://modules.prosody.im/mod_mam.html