Change to https for tigervnc URL

Our main web site now supports https, so update all references.
diff --git a/unix/vncconfig/vncconfig.man b/unix/vncconfig/vncconfig.man
index 06f9ca9..b685a46 100644
--- a/unix/vncconfig/vncconfig.man
+++ b/unix/vncconfig/vncconfig.man
@@ -114,7 +114,7 @@
 .BR vncserver (1),
 .BR Xvnc (1)
 .br
-http://www.tigervnc.org
+https://www.tigervnc.org
 
 .SH AUTHOR
 Tristan Richardson, RealVNC Ltd. and others.