doc/credits.md: Make links clickable

Thanks @magiraud
3 jobs for doc/credits in 2 minutes and 18 seconds (queued for 1 second)
Status Job ID Name Coverage
  Test Tools
passed #661445
test_tools

00:00:09

 
  Tgz Release
failed #661446
cidocker
build_doc

00:02:09

 
  Deploy Prod
skipped #661447
allowed to fail manual
deploy_doc
 
Name Stage Failure
failed
build_doc Tgz Release
  https://doi.org/10.1182/blood-2019-125028
http://dx.doi.org/10.1182/blood-2015-07-655159
http://dx.doi.org/10.1002/cyto.a.23604
http://dx.doi.org/10.1002/eji.201444985
https://dx.doi.org/10.1182/blood-2017-10-810739
https://doi.org/10.1002/pbc.27787
https://doi.org/10.1002/path.5376
https://www.ncbi.nlm.nih.gov/pubmed/22415752
ERROR: Job failed: exit code 1