Fix "session-management-for-emacs" license

- Change "license" to "licenses"
- Change to "gpl2", rather than non-existant "gpl" attr

pkgs/applications/editors/emacs/elisp-packages/session-management-for-emacs/default.nix

@@ -23,7 +23,7 @@ stdenv.mkDerivation rec {
     */
     description = "Small session management for emacs";
     homepage = "http://emacs-session.sourceforge.net/";
-    license = license.gpl;
+    license = licenses.gpl2;
     maintainers = with maintainers; [ ];
   };
 }