page_template: Fix a bug

Close processing-r/Processing.R#213

Signed-off-by: Ce Gao <ce.gao@outlook.com>

Author: gaocegege

templates/page_template.jinja

@@ -24,7 +24,7 @@
             <div id="navigation">
                 <div class="navBar" id="mainnav">
                     <a href="/">Cover</a><br/>
-                    <a href="reference/index.html">Reference</a><br/>
+                    <a href="/reference/">Reference</a><br/>
                     <!-- <a href="/tutorials/">Tutorials</a><br/>
                     <a href="/examples/">Examples</a><br/> -->
                     <a href="https://github.com/gaocegege/Processing.R/issues">Bugs</a>