[wwwdocs] Tweak hierarchy of headings in gcc-5/porting_to.html

[Gerald Pfeifer]

While working on the porting_to.html guides for later versions, I
noticed that the Links heading was at the wrong level (<h3> instead
of <h2>).

Fixed thusly; committed.

Gerald

Index: gcc-5/porting_to.html
===================================================================
RCS file: /cvs/gcc/wwwdocs/htdocs/gcc-5/porting_to.html,v
retrieving revision 1.14
diff -u -r1.14 porting_to.html
--- gcc-5/porting_to.html       1 Sep 2018 23:42:06 -0000       1.14
+++ gcc-5/porting_to.html       24 Sep 2018 21:53:57 -0000
@@ -428,7 +428,8 @@
 <code>return static_cast&lt;Y&amp;&gt;(y);</code>.
 </p>
 
-<h3>Links</h3>
+
+<h2>Links</h2>
 
 <p>
 Marek Polacek and Jakub Jelinek,