ATLAS: update of `atlas-higgs-challenge-2014.xml`

* Fixes typos, improves descriptions, amends author list, as discussed
  by email.  (addresses #660) (PR #751)

Co-authored-by: Tibor Simko <tibor.simko@cern.ch>
Signed-off-by: Tibor Simko <tibor.simko@cern.ch>