Add missing link for nix config

Author: bryango

docs/src/howto/build-tectonic/index.md

@@ -196,5 +196,6 @@ Tectonic's source repo. The nixpkgs definition also takes care of some
 packaging related issues, such as bundling with the correct version of `biber`
 to avoid [incompatibilities][biber-mismatch].
 
+[configured]: https://nixos.org/manual/nix/stable/command-ref/conf-file#conf-experimental-features
 [nixpkgs]: https://github.com/NixOS/nixpkgs
 [biber-mismatch]: https://github.com/tectonic-typesetting/tectonic/issues/893