doc: fix header level typo

PR-URL: https://github.com/nodejs/node/pull/14707
Refs: https://github.com/nodejs/node/pull/12756
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Daijiro Wachi <daijiro.wachi@gmail.com>
Reviewed-By: Sakthipriyan Vairamani <thechargingvolcano@gmail.com>

doc/api/cli.md

@@ -185,7 +185,7 @@ added: v8.0.0
 Enable loading native modules compiled with the ABI-stable Node.js API (N-API)
 (experimental).
 
-## `--abort-on-uncaught-exception`
+### `--abort-on-uncaught-exception`
 <!-- YAML
 added: v0.10
 -->