Replace bare directory reference with default.nix

Nix cannot refer to the current directory with only ./, that's a syntax error
not a path

Change-Id: Id1851e1b7fb3d68571d67bfbf1c13faabc4d47eb
diff --git a/flake.nix b/flake.nix
index 5f0f5fe..02d96d7 100644
--- a/flake.nix
+++ b/flake.nix
@@ -2,6 +2,6 @@
   description = "Clicks helpers for writing boilerplatey nix";
 
   outputs = { self, nixpkgs }: {
-    helpers = import ./;
+    helpers = import ./default.nix;
   };
 }