Fix documentation in Iterable.scala (#21692)

The documentation for `zipAll` is incorrect, e.g., `List(1, 2,
3).zipAll(Set("x"), 0, "")` produces a list, not a set.

Author: hamzaremmal

scala2-library-bootstrapped/src/scala/collection/Iterable.scala

@@ -756,7 +756,7 @@ trait IterableOps[+A, +CC[_], +C] extends Any with IterableOnce[A] with Iterable
     *  @param that     the iterable providing the second half of each result pair
     *  @param thisElem the element to be used to fill up the result if this $coll is shorter than `that`.
     *  @param thatElem the element to be used to fill up the result if `that` is shorter than this $coll.
-    *  @return        a new collection of type `That` containing pairs consisting of
+    *  @return        a new collection of the type of this $coll containing pairs consisting of
     *                 corresponding elements of this $coll and `that`. The length
     *                 of the returned collection is the maximum of the lengths of this $coll and `that`.
     *                 If this $coll is shorter than `that`, `thisElem` values are used to pad the result.