Added target="_blank" so doesn't take you away
Otherwise, you can lose this page.
<div class="preview container">
<p class="preview-text text-center">
Here's a non-interactive preview on
-<a href="https://nbviewer.jupyter.org">nbviewer</a>
+<a target="_blank" href="https://nbviewer.jupyter.org">nbviewer</a>
while we start a server for you.
Your binder will open automatically when it is ready.
</p>
