starspace: fix version

pkgs/applications/science/machine-learning/starspace/default.nix

···
 
 stdenv.mkDerivation rec {
   pname = "starspace";
-  version = "unstable-2021-01-17";
+  version = "unstable-2019-12-13";
 
   src = fetchFromGitHub {
     owner = "facebookresearch";