eval-config.nix: configuration->configuration.nix

`configuration` seems to be a reference to an argument that was
removed seven years ago in commit 2892aed7.

`configuration.nix` makes it a big more clear what we're referring to.

Changed files
+1 -1
nixos
+1 -1
nixos/lib/eval-config.nix
···
-
# From an end-user configuration file (`configuration'), build a NixOS
+
# From an end-user configuration file (`configuration.nix'), build a NixOS
# configuration object (`config') from which we can retrieve option
# values.