doc: fix err_synthetic issue on v11.x

Fixes: #25750

PR-URL: #25770
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Gireesh Punathil <gpunathi@in.ibm.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>

doc/api/errors.md

@@ -1690,7 +1690,7 @@ An attempt has been made to create a string longer than the maximum allowed
 length.
 
 <a id="ERR_SYNTHETIC"></a>
-#### ERR_SYNTHETIC
+### ERR_SYNTHETIC
 
 An artificial error object used to capture the call stack for diagnostic
 reports.