Update on mainframe the correct url for Unit (#842)

chore: Update on mainframe the correct url for Unit

Author: lamATnginx

layouts/index.html

@@ -404,7 +404,7 @@ <h1 class="homepage-item-text">
             More NGINX Products
         </div>
 
-        <a href="https://unit.nginx.org/" alt="NGINX Unit">
+        <a href="{{ .Site.BaseURL }}nginx-unit" alt="NGINX Unit">
         <div class="homepage-item">
             <div class="homepage-item-heading">
                 <div class="homepage-item-logo">