Apply suggestions from code review

Author: GromNaN

src/MongoDBServiceProvider.php

@@ -51,7 +51,7 @@ public function register()
                     $config['collection'] ?? 'cache',
                     $this->getPrefix($config),
                     $app['db']->connection($config['lock_connection'] ?? $config['connection'] ?? null),
-                    ($config['lock_collection'] ?? $config['collection'] ?? 'cache') . '_locks',
+                    $config['lock_collection'] ?? (($config['collection'] ?? 'cache') . '_locks'),
                     $config['lock_lottery'] ?? [2, 100],
                     $config['lock_timeout'] ?? 86400,
                 );