Fix confidential_note_events parameter documentation

Previously it incorrectly said it was a boolean type and it did not
have the "required" value.

doc/api/services.md

@@ -1253,7 +1253,7 @@ Parameters:
 | `confidential_issue_channel` | string | false | The name of the channel to receive confidential issues events notifications |
 | `merge_request_channel` | string | false | The name of the channel to receive merge request events notifications |
 | `note_channel` | string | false | The name of the channel to receive note events notifications |
-| `confidential_note_channel` | boolean | The name of the channel to receive confidential note events notifications |
+| `confidential_note_channel` | string | false | The name of the channel to receive confidential note events notifications |
 | `tag_push_channel` | string | false | The name of the channel to receive tag push events notifications |
 | `pipeline_channel` | string | false | The name of the channel to receive pipeline events notifications |
 | `wiki_page_channel` | string | false | The name of the channel to receive wiki page events notifications |