[stdlib] NFC: Fix typo in comment.

Author: barnard-b

stdlib/public/core/StringObject.swift

@@ -293,7 +293,7 @@ extension _StringObject.CountAndFlags {
  on arm64.
 */
 extension _StringObject.Nibbles {
-  // The canonical empty sting is an empty small string
+  // The canonical empty string is an empty small string
   @inlinable @inline(__always)
   internal static var emptyString: UInt64 {
     return _StringObject.Nibbles.small(isASCII: true)