doc: fix markdown syntax

Author: mle86

README.md

@@ -66,7 +66,7 @@ Interface methods
 which are documented in the [`WorkServerAdapter`](https://github.com/mle86/php-wq/blob/master/doc/Ref_WorkServerAdapter_interface.md) interface:
 
 * <code>public function <b>storeJob</b> (string $workQueue, Job $job, int $delay = 0)</code>
-* <code>public function <b>getNextQueueEntry</b> ($workQueue, int $timeout = DEFAULT_TIMEOUT): ?QueueEntry</code>
+* <code>public function <b>getNextQueueEntry</b> ($workQueue, int $timeout = DEFAULT\_TIMEOUT): ?QueueEntry</code>
 * <code>public function <b>buryEntry</b> (QueueEntry $entry)</code>
 * <code>public function <b>requeueEntry</b> (QueueEntry $entry, int $delay, string $workQueue = null)</code>
 * <code>public function <b>deleteEntry</b> (QueueEntry $entry)</code>