Update trainers.md
Update trainers.md

move heading

Former-commit-id: 88ed9f3d37d21ea2a6f65298bfaead25598e603b [formerly 848ebbf6e4ead939687f49905857bdbbd2b5d334]
Former-commit-id: 4c780c6097ce1cdae10ff07e9ec3a46f49ec986e
LibraryCarpentry/librarycarpentry.github.io,LibraryCarpentry/librarycarpentry.github.io,LibraryCarpentry/librarycarpentry.github.io