Change to https for tigervnc URL

Our main web site now supports https, so update all references.
diff --git a/unix/vncserver.man b/unix/vncserver.man
index 9108683..95f7960 100644
--- a/unix/vncserver.man
+++ b/unix/vncserver.man
@@ -192,7 +192,7 @@
 .BR vncconfig (1),
 .BR Xvnc (1)
 .br
-http://www.tigervnc.org
+https://www.tigervnc.org
 
 .SH AUTHOR
 Tristan Richardson, RealVNC Ltd., D. R. Commander and others.