Fix argument order and type var in Seq.fold_lefti documentation

`f` has type `int -> 'b -> 'a -> 'b`, therefore the index is the first
argument, and the accumulator is of type `'b`.

stdlib/seq.mli

@@ -188,15 +188,15 @@ val iteri : (int -> 'a -> unit) -> 'a t -> unit
     @since 4.14 *)
 
 val fold_lefti : (int -> 'b -> 'a -> 'b) -> 'b -> 'a t -> 'b
-(** [fold_lefti f _ xs] invokes [f _ i x] successively
+(** [fold_lefti f _ xs] invokes [f i _ x] successively
     for every element [x] located at index [i] of the sequence [xs].
 
-    An accumulator of type ['a] is threaded through the calls to [f].
+    An accumulator of type ['b] is threaded through the calls to [f].
 
     It terminates only if the sequence [xs] is finite.
 
     [fold_lefti f accu xs] is equivalent to
-    [fold_left (fun accu (i, x) -> f accu i x) accu (zip (ints 0) xs)].
+    [fold_left (fun accu (i, x) -> f i accu x) accu (zip (ints 0) xs)].
 
     @since 4.14 *)