Fix inconsistency in team_member.reduce sample code snippet

[mabruzzo]

This commit modifies the snippet illustrating how to use a method of the member_type type to trigger a reduction. There were 3 adjustments:

1. The name of the triggered method was invalid: the TeamHandleConcept concept doesn't specify a reduce method. I adjusted the name to instead reference the team_reduce method. (maybe the way it was written was correct at some point in an earlier version?)
2. Consequently, I also needed to adjust how the reduction was called
3. I fixed some inconsistent indentation within the snippet

There were 2 areas where I was a little uncertain on what to do:

• I couldn't tell if whether it was idiomatic to include the using Kokkos::Sum; statement. Let me know if I should remove it
• I was tempted to rewrite a() += team_sum; as the following snippet (inspired by the other code-snippet in the subsection), but I decided to ask before doing that

  if( team_member.team_rank() == 0 ) {
    a() += team_sum;
  }

[masterleinad]

Thanks!

[masterleinad]

I was tempted to rewrite a() += team_sum; as the following snippet (inspired by the other code-snippet in the subsection), but I decided to ask before doing that

Would you mind adding that as well, possibly using Kokkos::single or actually calling Kokkos::atomic_add?

[mabruzzo]

I was tempted to rewrite a() += team_sum; as the following snippet (inspired by the other code-snippet in the subsection), but I decided to ask before doing that

Would you mind adding that as well, possibly using Kokkos::single or actually calling Kokkos::atomic_add?

Done. Let me know if you want any other changes.