doc: fix typo in synchronous randomBytes example

The string template was closed after `${buf.length}`
causing a syntax error within the example.

PR-URL: https://github.com/nodejs/node/pull/5781


Reviewed-By: Michaël Zasso <mic.besace@gmail.com>
Reviewed-By: Benjamin Gruenbaum <benjamingr@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>