improve string concatenation

Co-authored-by: Ferdinand Thiessen <[email protected]>
Signed-off-by: mcnesium <[email protected]>

templates/rss.php

@@ -34,7 +34,7 @@
 		<atom:link href="<?php p($_['rssLink']); ?>" rel="self" type="application/rss+xml" />
 <?php foreach ($_['activities'] as $activity) { ?>
 		<item>
-			<guid isPermaLink="false"><?php p($_['rssLink']); echo "?id="; p($activity['activity_id']); ?></guid>
+			<guid isPermaLink="false"><?php p($_['rssLink'] . '?id=' . $activity['activity_id']); ?></guid>
 <?php if (!empty($activity['subject'])): ?>
 			<title><?php p(str_replace("\n", ' ', $activity['subject'])); ?></title>
 <?php endif; ?>