lib/fixed-points.nix: correct typo

Corrects a minor typo in the description of `fix`; namely that `fix` allows factoring out syntactic recursion from another function.

Changed files
+1 -1
lib
+1 -1
lib/fixed-points.nix
···
}
```
-
This is where `fix` comes in, it contains the syntactic that's not in `f` anymore.
+
This is where `fix` comes in, it contains the syntactic recursion that's not in `f` anymore.
```nix
nix-repl> fix = f: