Mention typo3-rector in the Upgrade Guide

[linawolf]

Thanks a lot for your contribution, just a small remark

[wolffc]

thanks @linawolf i have no Restructured Text Knowlage

[linawolf]

@wolffc We have a cheat sheet for you: https://docs.typo3.org/m/typo3/docs-how-to-document/main/en-us/WritingReST/CheatSheet.html#links