Remove reference to `riak2`

As mentioned by LnL in IRC, we

> used to have riak and riak2

But this seems no longer the case, hence the example should be updated.

nixos/modules/services/databases/riak.nix

@@ -20,7 +20,7 @@ in
 
       package = mkOption {
         type = types.package;
-        example = literalExample "pkgs.riak2";
+        example = literalExample "pkgs.riak";
         description = ''
           Riak package to use.
         '';