doc: fix err_synthetic issue on v11.x

Fixes: #25750 PR-URL: #25770 Reviewed-By: Anna Henningsen <[email protected]> Reviewed-By: Gireesh Punathil <[email protected]> Reviewed-By: Luigi Pinca <[email protected]> Reviewed-By: Colin Ihrig <[email protected]>

Author: sreepurnajasti

doc/api/errors.md

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