Use h4 for changelog months

Author: StephenBeirlaen

src/changes-backwards-compatible.apib

@@ -14,7 +14,7 @@ We list all backwards-compatible additions here. These are currently available i
 
 - We added `ids` to the filters on `teams.list`.
 
-### March 2023
+#### March 2023
 
 - We added `customField`,`timeTracking` and `department` as allowed types on `migrate.id`.
 - We added `credit_note_date_before` and `credit_note_date_after` to the filters on `creditNotes.list`.