use 'ar' not 'a'

stdlib/weak.mli

@@ -53,13 +53,13 @@ val set : 'a t -> int -> 'a option -> unit
    (full) pointer to [el]; [Weak.set ar n None] sets the [n]th
    cell of [ar] to empty.
    @raise Invalid_argument if [n] is not in the range
-   0 to {!Weak.length}[ a - 1].*)
+   0 to {!Weak.length}[ ar - 1].*)
 
 val get : 'a t -> int -> 'a option
 (** [Weak.get ar n] returns None if the [n]th cell of [ar] is
    empty, [Some x] (where [x] is the value) if it is full.
    @raise Invalid_argument if [n] is not in the range
-   0 to {!Weak.length}[ a - 1].*)
+   0 to {!Weak.length}[ ar - 1].*)
 
 val get_copy : 'a t -> int -> 'a option
 (** [Weak.get_copy ar n] returns None if the [n]th cell of [ar] is
@@ -70,7 +70,7 @@ val get_copy : 'a t -> int -> 'a option
    the incremental GC from erasing the value in its current cycle
    ([get] may delay the erasure to the next GC cycle).
    @raise Invalid_argument if [n] is not in the range
-   0 to {!Weak.length}[ a - 1].
+   0 to {!Weak.length}[ ar - 1].
 
    If the element is a custom block it is not copied.
 
@@ -88,7 +88,7 @@ val fill : 'a t -> int -> int -> 'a option -> unit
 (** [Weak.fill ar ofs len el] sets to [el] all pointers of [ar] from
    [ofs] to [ofs + len - 1].
    @raise Invalid_argument
-   if [ofs] and [len] do not designate a valid subarray of [a].*)
+   if [ofs] and [len] do not designate a valid subarray of [ar].*)
 
 val blit : 'a t -> int -> 'a t -> int -> int -> unit
 (** [Weak.blit ar1 off1 ar2 off2 len] copies [len] weak pointers