ledger-2.6.3: rename the derivation to "ledger2" to allow installing this version instead of ledger 3.x (which doesn't support reading GNUCash files)

Discussed in <https://github.com/NixOS/nixpkgs/pull/2305>.

pkgs/applications/office/ledger/2.6.3.nix

···
 { stdenv, fetchurl, emacs, gmp, pcre, expat }:
 
 stdenv.mkDerivation rec {
-  name = "ledger-2.6.3";
+  name = "ledger2-2.6.3";
 
   src = fetchurl {
     url = "https://github.com/downloads/ledger/ledger/${name}.tar.gz";