Fixes #1546 Var name mistake

admin/themes/default/template/updates_pwg.tpl

@@ -127,7 +127,7 @@ p.release .errors {margin:0}
 </p>
 <p>
   {'A new version of Piwigo is available.'|@translate}<br>
-  {'This is a major update, with <a href="%s">new exciting features</a>.'|@translate:$RELEASE_URL} {'Some themes and plugins may be not available yet.'|@translate}
+  {'This is a major update, with <a href="%s">new exciting features</a>.'|@translate:$MAJOR_RELEASE_URL} {'Some themes and plugins may be not available yet.'|@translate}
 </p>
 <form action="" method="post">
 
@@ -166,4 +166,4 @@ p.release .errors {margin:0}
 
 <p><input type="hidden" name="upgrade_to" value="{$UPGRADE_TO}"></p>
 </form>
-{/if}
+{/if}