Change 302 redirect to 301

Change "302 Found" response to "301 Moved Permanently", as "302 Found" is meant for temporary redirects.

Was asked to do so in this comment: https://github.com/pi-hole/pi-hole/pull/1297#issuecomment-284335421

advanced/index.php

@@ -8,7 +8,7 @@ $serverName = escapeshellcmd($_SERVER['SERVER_NAME']);
 // Let's be nice and redirect them.
 if ($serverName === 'pi.hole')
 {
-    header('HTTP/1.1 302 Found');
+    header('HTTP/1.1 301 Moved Permanently');
     header("Location: /admin/");
 }