message: Fix issue template to mention doctest instead of Catch
diff: @@ -13,7 +13,7 @@ want it.
  This is only relevant for bug reports, but if you do have one,
  please provide a minimal set of steps to reproduce the problem.
  -Usually this means providing a small and self-contained code using Catch
  +Usually this means providing a small and self-contained code using doctest
  and specifying compiler flags/tools used if relevant.
  -->
  