Fix typo (#4620)

Author: philipturner

Darwin/Foundation-swiftoverlay/NSStringAPI.swift

@@ -205,7 +205,7 @@ extension String {
   ///
   /// - Parameters:
   ///   - bytes: A sequence of bytes to interpret using `encoding`.
-  ///   - encoding: The ecoding to use to interpret `bytes`.
+  ///   - encoding: The encoding to use to interpret `bytes`.
   public init?<S: Sequence>(bytes: __shared S, encoding: Encoding)
   where S.Iterator.Element == UInt8 {
     let byteArray = Array(bytes)