libexpr: Fix manual link in error message

It was changed to the old manual in 8895fa70a4

src/libexpr/eval.cc

@@ -1590,7 +1590,7 @@ void EvalState::autoCallFunction(Bindings & args, Value & fun, Value & res)
 Nix attempted to evaluate a function as a top level expression; in
 this case it must have its arguments supplied either by default
 values, or passed explicitly with '--arg' or '--argstr'. See
-https://nixos.org/manual/nix/stable/#ss-functions.)", symbols[i.name]);
+https://nixos.org/manual/nix/stable/expressions/language-constructs.html#functions.)", symbols[i.name]);
 
             }
         }