Fixes example snippet in ExistentialCollection.swift (swiftlang#31462)

* Fixes example snippet in ExistentialCollection.swift

* Removes extra parentheses

Author: valeriyvan

stdlib/public/core/ExistentialCollection.swift

@@ -51,7 +51,7 @@ public struct AnyIterator<Element> {
   ///     func digits() -> AnyIterator<String> {
   ///         let lazyStrings = (0..<10).lazy.map { String($0) }
   ///         let iterator:
-  ///             LazyMapIterator<IndexingIterator<Range<Int>>, String>
+  ///             LazyMapSequence<Range<Int>, String>.Iterator
   ///             = lazyStrings.makeIterator()
   ///
   ///         return AnyIterator(iterator)