sidebar: convert <</>> into brackets in page.how

We already do this in the license bodies, and we need to
match here (currently we just print junk like `replace <>
with ...` because we end up with garbled HTML.

This replacement should arguably be factored out, as it
would want to be the same everywhere (and we might want to
make it bold everywhere or something).

_includes/sidebar.html

@@ -4,7 +4,7 @@
     <div class='how-to-apply callout'>
       <h5>How to apply this license</h5>
       <p>
-        {{ page.how }}
+        {{ page.how | replace:"<<","[" | replace:">>","]" }}
       </p>
       {% if page.note %}
       <p class="note">