texlive manual: adds import of nixpkgs in nix-repl

I wanted to list the different texlive collections using the nix-repl, as per the [manual](https://nixos.org/nixpkgs/manual/#idm140737316065984). 

It didn't work, since the nixpkgs were not loaded. Doing `:l <nixpkgs>` first resolved the problem.

This change adds the nixpkgs loading step to the manual so that the next inexperienced person don't have to figure out why it didn't work.

I tested this on NixOS unstable (16.09pre90254.6b20d5b7) with nix-repl 1.11.3.