title: In gem server "rdoc not installed" but have rdoc in terminal
body: <p>When I gem server and go to the page, I notice there wasn't a link to the <a href="http://en.wikipedia.org/wiki/RDoc" rel="nofollow">RDoc</a>, and when I move to there a pop out says RDoc is not installed. </p>
  
  <p>But when I type <code>rdoc --help</code> in terminal, there is a reply. How to get to the rdoc, because I need to read <a href="https://github.com/intridea/omniauth" rel="nofollow">omniauth</a> rdoc?</p>