Update the current working APC docs

The previous docs link isn't found on the php.net website but there is docs for APC
so this patch just updates to the current valid docs on the website.

Author: Derick Alangi

_posts/14-02-01-Opcode-Cache.md

@@ -23,7 +23,7 @@ Read more about opcode caches:
 
 
 [opcache-book]: https://secure.php.net/book.opcache
-[APC]: https://secure.php.net/book.apc
+[APC]: https://www.php.net/book.apcu
 [XCache]: https://xcache.lighttpd.net/
 [Zend Optimizer+]: https://github.com/zendtech/ZendOptimizerPlus
 [WinCache]: https://www.iis.net/downloads/microsoft/wincache-extension