taskwarrior: improve meta.description (taskwarrior has nothing to do with GTD)

pkgs/applications/misc/taskwarrior/default.nix

···
   '';
 
   meta = with stdenv.lib; {
-    description = "GTD (getting things done) implementation";
+    description = "Highly flexible command-line tool to manage TODO lists";
     homepage = http://taskwarrior.org;
     license = licenses.mit;
     maintainers = with maintainers; [ marcweber jgeerds ];