message: Update concepts.hpp
  the example function should be auto
diff: @@ -618,7 +618,7 @@ CPP_PP_IGNORE_CXX2A_COMPAT_BEGIN
  // CPP_auto_fun
  // Usage:
  // template <typename A, typename B>
  -// void CPP_auto_fun(foo)(A a, B b)([const]opt [noexcept(true)]opt)opt
  +// auto CPP_auto_fun(foo)(A a, B b)([const]opt [noexcept(true)]opt)opt
  // (
  // return a + b
  // )
  