doc: fix Error property markdown level

Properties should be documented at same level as their constructor, as
they are in the other API doc sections.

PR-URL: #15247
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Timothy Gu <timothygu99@gmail.com>
Reviewed-By: Tobias Nießen <tniessen@tnie.de>
Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>

Author: sam-github

doc/api/errors.md

@@ -256,14 +256,14 @@ will affect any stack trace captured *after* the value has been changed.
 If set to a non-number value, or set to a negative number, stack traces will
 not capture any frames.
 
-#### error.code
+### error.code
 
 * {string}
 
 The `error.code` property is a string label that identifies the kind of error.
 See [Node.js Error Codes][] for details about specific codes.
 
-#### error.message
+### error.message
 
 * {string}