Change to https for tigervnc URL

Our main web site now supports https, so update all references.
diff --git a/vncviewer/vncviewer.man b/vncviewer/vncviewer.man
index 421290f..8762dfb 100644
--- a/vncviewer/vncviewer.man
+++ b/vncviewer/vncviewer.man
@@ -320,7 +320,7 @@
 .BR vncconfig (1),
 .BR vncserver (1)
 .br
-http://www.tigervnc.org
+https://www.tigervnc.org
 
 .SH AUTHOR
 Tristan Richardson, RealVNC Ltd. and others.