doc: make stream.Readable consistent

Rest of the document uses `stream.Readable` or `stream.Writable`. This
patch makes the usage consistent in the parameter description.

PR-URL: nodejs#16786
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Gireesh Punathil <gpunathi@in.ibm.com>
Reviewed-By: Joyee Cheung <joyeec9h3@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>

doc/api/stream.md

@@ -316,7 +316,7 @@ reader.pipe(writer);
 added: v0.9.4
 -->
 
-* `src` {[Readable][] Stream} The source stream that
+* `src` {stream.Readable} The source stream that
   [unpiped][`stream.unpipe()`] this writable
 
 The `'unpipe'` event is emitted when the [`stream.unpipe()`][] method is called