Re-enable `typos` hook with `exampledir/` correctly excluded

This effectively reverts commit 28d1f014aa.

.typos.toml

@@ -1,2 +1,5 @@
 [default.extend-words]
 als = "als"
+
+[files]
+extend-exclude = ["exampledir/*"]

flake.nix

@@ -114,7 +114,7 @@
             hooks = {
               nixfmt.enable = true;
               rustfmt.enable = true;
-              typos.enable = false;
+              typos.enable = true;
             };
           };
         };