Update about_mastering.md (remove Trainers)

[tkimnguyen]

I'm suggesting removing the Trainers section. It contains bios of past trainers, most of whom are no longer giving this training and some of whom are no longer involved with the community.

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[stevepiercy]

@tkimnguyen this one is not my decision to make. I've requested reviews from @ksuess and @pbauer for guidance.

Alternatively, to Trainers, I'd suggest Credits, Authors, or Maintainers as those have an historical context, instead of being current.

[tkimnguyen]

"Credits, Authors, or Maintainers" does sound better. The current wording gives the appearance of there being only a few trainers (those listed), and we know there were many more people who gave training based on this material.

[stevepiercy]

"Credits, Authors, or Maintainers" does sound better.

Its title could also be one or two of the groups, not all three. Whatever works.