doc: add missing heading for error

PR-URL: #15325
Reviewed-By: Richard Lau <riclau@uk.ibm.com>
Reviewed-By: Rich Trott <rtrott@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Tobias Nießen <tniessen@tnie.de>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>

doc/api/errors.md

@@ -680,6 +680,7 @@ An invalid HTTP informational status code has been specified. Informational
 status codes must be an integer between `100` and `199` (inclusive).
 
 <a id="ERR_HTTP2_INVALID_PACKED_SETTINGS_LENGTH"></a>
+### ERR_HTTP2_INVALID_PACKED_SETTINGS_LENGTH
 
 Input `Buffer` and `Uint8Array` instances passed to the
 `http2.getUnpackedSettings()` API must have a length that is a multiple of