doc: fix header level for crypto.constants

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

PR-URL: #9187
Reviewed-By: Colin Ihrig <[email protected]>
Reviewed-By: Anna Henningsen <[email protected]>
Reviewed-By: James M Snell <[email protected]>

Author: evanlucas

doc/api/crypto.md

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