Fix doc comment of DiscontiguousSlice.swift to make it match the code (#82220)

Author: valeriyvan

stdlib/public/core/DiscontiguousSlice.swift

@@ -350,7 +350,7 @@ extension DiscontiguousSlice where Base: MutableCollection {
   /// past the last element of a collection, so it doesn't correspond with an
   /// element.
   ///
-  /// - Parameter position: The position of the element to access. `position`
+  /// - Parameter i: The position of the element to access. `i`
   ///   must be a valid index of the collection that is not equal to the
   ///   `endIndex` property.
   ///