Update index.html

s/http/https/ for docbook guide

Author: daobrien

index.html

@@ -54,7 +54,7 @@ <h3>
     </h3>
 
       <h3>
-        <a href="http://stylepedia.net/docbook/4.0" title="DocBook XML Guide v4.0">
+        <a href="https://stylepedia.net/docbook/4.0" title="DocBook XML Guide v4.0">
           <img class="social" src="font/fontawesome/svgs/solid/code.svg" alt="XML Guide" width="40em" /> RHT DocBook XML Guide v4.0</a>
       </h3>