docs: Fix reference to ValidatorConfig documentation

Change-Id: I43d96c64d031ad318333de8613e589633e58b52e
refs: #4184
diff --git a/nfd.conf.sample.in b/nfd.conf.sample.in
index 7e4c940..f27c6ef 100644
--- a/nfd.conf.sample.in
+++ b/nfd.conf.sample.in
@@ -280,9 +280,9 @@
   ; {
   ;   ; This section defines the trust model for NFD RIB Management. It consists of rules and
   ;   ; trust-anchors, which are briefly defined in this file.  For more information refer to
-  ;   ; manpage of ndn-validator.conf:
+  ;   ; validator configuration file format documentation:
   ;   ;
-  ;   ;     man ndn-validator.conf
+  ;   ;    https://named-data.net/doc/ndn-cxx/current/tutorials/security-validator-config.html
   ;   ;
   ;   ; A trust-anchor is a pre-trusted certificate.  This can be any certificate that is the
   ;   ; root of certification chain (e.g., NDN testbed root certificate) or an existing