Outdated nixpkgs link on the manual

[nico202]

The hydra manual [1] points to an svn nixpkgs repository [2]. It (accordingly to @grahamc on irc) has not been updated for years. (The checkout fail because of wrong ssl certificate)

[1] https://nixos.org/hydra/manual/#idm140737315928400
[2] https://svn.nixos.org/repos/nix/nixpkgs/trunk

[invokesus]

The manual still points to the subversion repo even though the xml file has been updated.