Fix extended regular expressions reference URL in advanced.md
The original URL started with https://www.opengroup.org and fails to redirect to the reference page. According to Chrome's dev tools (Network tab), the response to that request is a HTTP 302 Found, with the new location being "balancer://pubs/onlinepubs/
009695399/basedefs/xbd_chap09.html". Chrome does not know what to do with that and displays a blank page.
Changing the `www` subdomain to `pubs` results in HTTP 200.
PiperOrigin-RevId:
721804738
Change-Id: I816d08d5baf957c63ac97506d04c7cc49c3a5ce7