Fix param typo in notes.md

doc/api/notes.md

@@ -98,7 +98,7 @@ POST /projects/:id/issues/:issue_iid/notes
 Parameters:
 
 - `id` (required) - The ID or [URL-encoded path of the project](README.md#namespaced-path-encoding)
-- `issue_id` (required) - The IID of an issue
+- `issue_iid` (required) - The IID of an issue
 - `body` (required) - The content of a note
 - `created_at` (optional) - Date time string, ISO 8601 formatted, e.g. 2016-03-11T03:45:40Z (requires admin or project/group owner rights)