libgit2: correct license

From libgit2's COPYING file:

    Note that the only valid version of the GPL as far as this project
    is concerned is _this_ particular version of the license (ie v2, not
    v2.2 or v3.x or whatever), unless explicitly otherwise stated.

- 6c5520f334/COPYING (L4-L6)

pkgs/development/libraries/libgit2/default.nix

@@ -76,7 +76,7 @@ stdenv.mkDerivation rec {
     description = "Linkable library implementation of Git that you can use in your application";
     mainProgram = "git2";
     homepage = "https://libgit2.org/";
-    license = licenses.gpl2Plus;
+    license = licenses.gpl2Only;
     platforms = platforms.all;
     maintainers = with maintainers; [ SuperSandro2000 ];
   };