doc: Typo in buffer.markdown referencing buf.write()

The buffer's write function is documented below the
buf.toString function and all of the docs reference
"buf" instead of "buffer".

PR-URL: nodejs#4324
Reviewed-By: James M Snell <jasnell@gmail.com>

doc/api/buffer.markdown

@@ -611,7 +611,7 @@ defaults to `'utf8'`. The `start` and `end` parameters default to `0` and
     buf.toString('utf8',0,5); // outputs: abcde
     buf.toString(undefined,0,5); // encoding defaults to 'utf8', outputs abcde
 
-See `buffer.write()` example, above.
+See `buf.write()` example, below.
 
 
 ### buf.toJSON()