message: Adjust docs to modern release tagging scheme
diff: The latest released version of doctest can be obtained from here: https://raw.githubusercontent.com/doctest/doctest/master/doctest/doctest.h
  -You can substitute ```master``` with ```dev``` or a tag like ```1.2.9``` for a specific version in the URL above.
  +You can substitute ```master``` with ```dev``` or a tag like ```v1.4.8``` for a specific version in the URL above.
  ### CMake
  