Replace jenkins broken links (incorrect job name)

From README.rst

.. image:: https://jenkins.debian.net/buildStatus/icon?job=reproducible_diffoscope_from_git_master&plastic=true
  :target: https://jenkins.debian.net/job/reproducible_diffoscope_from_git_master

which result in image/badge: [build 'not run']

because the job name is reproducible_diffoscope_from_git_master instead of reproducible_diffoscope_from_git

fix: for image use: (plastic=true doesn't change response so possible to remove this parameter?)

https://jenkins.debian.net/buildStatus/icon?job=reproducible_diffoscope_from_git

for target use:

https://jenkins.debian.net/job/reproducible_diffoscope_from_git/

or maybe better: (in menu can choose other diffoscope items)

https://jenkins.debian.net/view/reproducible/view/diffoscope/job/reproducible_diffoscope_from_git/

which will result in image/badge: [build 'passing']