doc: make writable.setDefaultEncoding() return this

Let this function return `this` for parity with `readable.setEncoding()`.

PR-URL: #5040
Fixes: #5013

Reviewed-By: Matteo Collina <matteo.collina@gmail.com>
Reviewed-By: Calvin Metcalf <calvin.metcalf@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>

Author: estliberitas

doc/api/stream.md

@@ -695,6 +695,7 @@ file.end('world!');
 #### writable.setDefaultEncoding(encoding)
 
 * `encoding` {String} The new default encoding
+* Return: `this`
 
 Sets the default encoding for a writable stream.
 

lib/_stream_writable.js

@@ -251,6 +251,7 @@ Writable.prototype.setDefaultEncoding = function setDefaultEncoding(encoding) {
   if (!Buffer.isEncoding(encoding))
     throw new TypeError('Unknown encoding: ' + encoding);
   this._writableState.defaultEncoding = encoding;
+  return this;
 };
 
 function decodeChunk(state, chunk, encoding) {