Fix broken Flattr link (#14677)

README.md

 
 The current Marlin dev team consists of:
 
- - Scott Lahteine [[@thinkyhead](https://github.com/thinkyhead)] - USA   [![Flattr Scott](http://api.flattr.com/button/flattr-badge-large.png)](https://flattr.com/submit/auto?user_id=thinkhead&url=https://github.com/MarlinFirmware/Marlin&title=Marlin&language=&tags=github&category=software)
+ - Scott Lahteine [[@thinkyhead](https://github.com/thinkyhead)] - USA   [![Flattr Scott](http://api.flattr.com/button/flattr-badge-large.png)](https://flattr.com/submit/auto?user_id=thinkyhead&url=https://github.com/MarlinFirmware/Marlin&title=Marlin&language=&tags=github&category=software)
  - Roxanne Neufeld [[@Roxy-3D](https://github.com/Roxy-3D)] - USA
  - Bob Kuhn [[@Bob-the-Kuhn](https://github.com/Bob-the-Kuhn)] - USA
  - Chris Pepper [[@p3p](https://github.com/p3p)] - UK