[Standards] Small fixes for XEP-0424

Hi all, I opened a pull request <https://github.com/xsf/xeps/pull/1326> to contribute small fixes to XEP-0424: - Fixes the schema of the |<retracted>| element: * The is no 'from' attribute * There is a mandatory 'id' attribute * 'by' and 'stamp' are not mandatory - Add a fallback 'for' attribute in Example 4. I hope that's useful. Cheers, -- Nicolas