Update manage_cache.php

tagline copied from mail

Reviewed by: epriestley

See: <phacility#446>

Author: dzhibas

scripts/cache/manage_cache.php

@@ -5,7 +5,7 @@
 require_once $root.'/scripts/__init_script__.php';
 
 $args = new PhutilArgumentParser($argv);
-$args->setTagline('manage mail');
+$args->setTagline('manage cache');
 $args->setSynopsis(<<<EOSYNOPSIS
 **cache** __command__ [__options__]
     Manage Phabricator caches.