springboot Update link to new Spring Security docs location

Hi,

I found yet some more links to be broken in the latest snapshot docs. Apparently, the Spring-Security docs only exist under /html5/ instead of /htmlsingle/ as of 5.3.x.

Cheers, Christoph

Comment From: philwebb

See https://github.com/spring-projects/spring-security/issues/8420 for discussion about why html5 is being used.

Comment From: snicoll

Unfortunately, it doesn't look like the Spring Security team wants to reconsider the use of html5 but they've put a redirect in place so that previous milestones now work. Thanks for spotting that @dreis2211!