doc: use lib.cleanSource

Otherwise, running "nix-build" in the doc directory would create a
result symlink, so running "nix-build" again would produce a different
derivation, and so on forever, because the result symlink would keep
changing and wasn't ignored.

doc/default.nix

@@ -15,7 +15,7 @@ in pkgs.stdenv.mkDerivation {
     xmlformat
   ];
 
-  src = ./.;
+  src = lib.cleanSource ./.;
 
   makeFlags = [
     "PANDOC_LUA_FILTERS_DIR=${pkgs.pandoc-lua-filters}/share/pandoc/filters"