doc: fix stream `_writev` header size

PR-URL: https://github.com/iojs/io.js/pull/916
Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>

doc/api/stream.markdown

@@ -1067,7 +1067,7 @@ the class that defines it, and should not be called directly by user
 programs.  However, you **are** expected to override this method in
 your own extension classes.
 
-### writable.\_writev(chunks, callback)
+#### writable.\_writev(chunks, callback)
 
 * `chunks` {Array} The chunks to be written.  Each chunk has following
   format: `{ chunk: ..., encoding: ... }`.