doc: fix typo in fs writeSync param list

Rename `buffer` to `data` in param list of
fs.writeSync(fd, data[, position[, encoding]])

PR-URL: nodejs#5984
Reviewed-By: Brian White <mscdex@mscdex.net>
Reviewed-By: James M Snell <jasnell@gmail.com>

doc/api/fs.markdown

@@ -1342,7 +1342,7 @@ The synchronous version of [`fs.writeFile()`][]. Returns `undefined`.
 ## fs.writeSync(fd, data[, position[, encoding]])
 
 * `fd` {Integer}
-* `buffer` {String | Buffer}
+* `data` {String | Buffer}
 * `position` {Integer}
 * `encoding` {String}