URL ref for filter_variable.cpp is stale.

[grahamwhaley]

Description

Minor - one of the URL refs in the code base has gone stale. I wonder if we can update it.

Steps To Reproduce Problem

The link http://www.musicdsp.org/showArchiveComment.php?ArchiveID=92 in filter_variable.cpp looks to have gone stale, no longer exists, and in fact fails to return an error or render in my browser :-(

It looks like that web site maybe got a rewrite, and even possibly the old refs and information no long exist?

I suspect the link would now be one of:

• https://www.musicdsp.org/en/latest/Filters/23-state-variable.html
• https://www.musicdsp.org/en/latest/Filters/142-state-variable-filter-chamberlin-version.html

but, I can't quite figure out if either those matches the original link etc., or I would have submitted a PR.