Fix flattr link

Co-Authored-By: MarcelMo <marcelmo@users.noreply.github.com>
3 jobs for 1.1.x
Status Job ID Name Coverage
  Build
failed #136
build

 
  Test
skipped #138
allowed to fail
code_quality
skipped #137
test
 
Name Stage Failure
failed
build Build There has been a timeout failure or the job got stuck. Check your timeout limits or try again
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