docs: mark secret as (required) in example.config.toml

Follow-up to #503, which introduced the (required) marker on bind-to.
secret is the other top-level option without a sensible default, so it
takes the same marker, on the first line of its description.

Symmetric with #504 ((default) on prefer-ipv6). Rolling the convention
out to the rest of the file is left for separate PRs.

example.config.toml

 # you have any issue.
 debug = true
 
-# A secret. Please remember that mtg supports only FakeTLS mode, legacy
-# simple and secured mode are prohibited. For you it means that secret
-# should either be base64-encoded or starts with ee.
+# A secret (required). Please remember that mtg supports only FakeTLS
+# mode, legacy simple and secured mode are prohibited. For you it means
+# that secret should either be base64-encoded or starts with ee.
 secret = "ee367a189aee18fa31c190054efd4a8e9573746f726167652e676f6f676c65617069732e636f6d"
 
 # Host:port pair to run proxy on (required).