correction in home page join button

Signed-off-by: Toshaan Bharvani <toshaan@vantosh.com>

themes/openpowerfoundation/layouts/partials/home-promo.html

@@ -3,6 +3,6 @@
   <div class="promo__container container text-center">
     <h1>{{ .Param "promo-h1" }}</h1>
     <p>{{ .Param "promo-p1" }}</p>
-    <p><a class="btn btn-light btn-outline-primary" href="/join">{{ .Param "promo-cta" }}</a></p>
+    <p><a class="btn btn-light btn-outline-primary" href="{{ "/join/" | relURL }}">{{ .Param "promo-cta" }}</a></p>
   </div>
 </section>