Doygen: remove wrong usage of \example directive.

For whatever reason it produced a file like
_2tmp_2reprotest_81JBnX9_2build-experiment-1_2build-experiment-1_2opm_2output_2OutputWriter_8hpp-example.html
with the full path in it.

BTW: correct usage is "\example <cpp-file>" where the cpp file
includes the example code.

opm/output/OutputWriter.hpp

@@ -42,7 +42,6 @@ struct PhaseUsage;
  * Use the create() function to setup a chain of writer based on the
  * configuration values, e.g.
  *
- * \example
  * \code{.cpp}
  *  ParameterGroup params (argc, argv, false);
  *  auto parser = std::make_shared <const Deck> (