doc: use consistent header typography

Adding backticks to an entry that has them missing in the header, unlike
the other entries.

PR-URL: #35030
Reviewed-By: Derek Lewis <DerekNonGeneric@inf.is>
Reviewed-By: Michaël Zasso <targos@protonmail.com>
Reviewed-By: Anna Henningsen <anna@addaleax.net>

doc/api/errors.md

@@ -1929,7 +1929,7 @@ An attempt was made to issue Server Name Indication from a TLS server-side
 socket, which is only valid from a client.
 
 <a id="ERR_TLS_PSK_SET_IDENTIY_HINT_FAILED"></a>
-### ERR_TLS_PSK_SET_IDENTIY_HINT_FAILED
+### `ERR_TLS_PSK_SET_IDENTIY_HINT_FAILED`
 
 Failed to set PSK identity hint. Hint may be too long.