[stdlib] Use double space as sentence separator in IteratorType.next() docs.

stdlib/public/core/StringUnicodeScalarView.swift

@@ -163,7 +163,7 @@ extension String {
       }
 
       /// Advance to the next element and return it, or `nil` if no next element
-      /// exists. Once `nil` has been returned, all subsequent calls return `nil`.
+      /// exists.  Once `nil` has been returned, all subsequent calls return `nil`.
       public mutating func next() -> UnicodeScalar? {
         var result: UnicodeDecodingResult
         if _baseSet {