doc: fix title level at tls.md

PR-URL: #8782
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Teddy Katz <teddy.katz@gmail.com>
Reviewed-By: Gibson Fahnestock <gibfahn@gmail.com>

Author: yorkie

doc/api/tls.md

@@ -1191,7 +1191,7 @@ As with checking for the server [`secureConnection`](#tls_event_secureconnection
 event, `pair.cleartext.authorized` should be inspected to confirm whether the
 certificate used is properly authorized.
 
-## tls.createSecurePair([context][, isServer][, requestCert][, rejectUnauthorized][, options])
+### tls.createSecurePair([context][, isServer][, requestCert][, rejectUnauthorized][, options])
 <!-- YAML
 added: v0.3.2
 deprecated: v0.11.3