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

Author: Abseil Team

docs/advanced.md

@@ -514,7 +514,7 @@ TEST_F(FooDeathTest, DoesThat) {
 When built with Bazel and using Abseil, GoogleTest uses the
 [RE2](https://github.com/google/re2/wiki/Syntax) syntax. Otherwise, for POSIX
 systems (Linux, Cygwin, Mac), GoogleTest uses the
-[POSIX extended regular expression](https://www.opengroup.org/onlinepubs/009695399/basedefs/xbd_chap09.html#tag_09_04)
+[POSIX extended regular expression](https://pubs.opengroup.org/onlinepubs/009695399/basedefs/xbd_chap09.html#tag_09_04)
 syntax. To learn about POSIX syntax, you may want to read this
 [Wikipedia entry](https://en.wikipedia.org/wiki/Regular_expression#POSIX_extended).