docs: add links

doc/api/fs.markdown

@@ -418,7 +418,7 @@ Objects returned from `fs.stat()` and `fs.lstat()` are of this type.
 
 ## fs.ReadStream
 
-`ReadStream` is a `Readable Stream`.
+`ReadStream` is a [Readable Stream](streams.html#readable_Stream).
 
 ### Event: 'open'
 
@@ -450,7 +450,7 @@ An example to read the last 10 bytes of a file which is 100 bytes long:
 
 ## fs.WriteStream
 
-`WriteStream` is a `Writable Stream`.
+`WriteStream` is a [Writable Stream](streams.html#writable_Stream).
 
 ### Event: 'open'