doc/general.texi: update AviSynth+ reference page

Directed to the AviSynth+ entry on AviSynth Wiki rather than to
the github repository, since the wiki page is both more informative
and has the relevant Git/download links.  The github releases page
is little more than a changelog.

doc/general.texi

@@ -46,7 +46,7 @@ without needing to search for these headers themselves.
 
 For Windows, supported AviSynth variants are
 @url{http://avisynth.nl, AviSynth 2.6 RC1 or higher} for 32-bit builds and
-@url{http://avs-plus.net, AviSynth+ r1718 or higher} for 32-bit and 64-bit builds.
+@url{http://avisynth.nl/index.php/AviSynth+, AviSynth+ r1718 or higher} for 32-bit and 64-bit builds.
 
 For Linux and OS X, the supported AviSynth variant is
 @url{https://github.com/avxsynth/avxsynth, AvxSynth}.