doc: fix header level for crypto.constants

The header level for crypto.constants was off by one.

PR-URL: https://github.com/nodejs/node/pull/9187
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: James M Snell <jasnell@gmail.com>

doc/api/crypto.md

@@ -992,7 +992,7 @@ thrown.
 
 ## `crypto` module methods and properties
 
-## crypto.constants
+### crypto.constants
 <!-- YAML
 added: v6.3.0
 -->