Change to https for tigervnc URL

Our main web site now supports https, so update all references.
diff --git a/unix/vncpasswd/vncpasswd.man b/unix/vncpasswd/vncpasswd.man
index a62c0ed..9e68181 100644
--- a/unix/vncpasswd/vncpasswd.man
+++ b/unix/vncpasswd/vncpasswd.man
@@ -47,7 +47,7 @@
 .BR Xvnc (1)
 .BR vncconfig (1),
 .br
-http://www.tigervnc.org
+https://www.tigervnc.org
 
 .SH AUTHORS
 Tristan Richardson, RealVNC Ltd., Antoine Martin, D. R. Commander and others.