Update link to contributing page

As the wiki page only links to the contributing page on the website, it makes sense to directly link there :)

templates/index.html

@@ -134,7 +134,7 @@ postmarketOS // real Linux distribution for phones
                 <a href="/install/">Try it out</a>
             </div>
             <div class="inline-button">
-                <a href="https://wiki.postmarketos.org/wiki/Contributing">Get involved</a>
+                <a href="https://postmarketos.org/contribute/">Get involved</a>
             </div>
         </div>
     </section>