doc: fix incorrect heading level

PR-URL: https://github.com/nodejs/node/pull/35965
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Antoine du Hamel <duhamelantoine1995@gmail.com>
Reviewed-By: Richard Lau <rlau@redhat.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>

doc/api/dns.md

@@ -950,7 +950,7 @@ Here is an example of the result object:
     minttl: 60 } ]
 ```
 
-## `dnsPromises.resolveCaa(hostname)`
+### `dnsPromises.resolveCaa(hostname)`
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