transmission: settings doc location changed

(cherry picked from commit 172ca5884be674452c105285cbd2308ca6ca31e6)

nixos/modules/services/torrent/transmission.nix

···
           time the service starts). String values must be quoted, integer and
           boolean values must not.
 
-          See https://trac.transmissionbt.com/wiki/EditConfigFiles for
-          documentation.
+          See https://github.com/transmission/transmission/wiki/Editing-Configuration-Files
+          for documentation.
         '';
       };