exercism-matrix: tweak instructions

[ErikSchierboom]

• The rot270 and rot90 tasks look to be somewhat random, as in: they don't really fit in the story. Maybe we convert them to a task that has students flip the matrix (by rotating it 180 degrees), which can then be use in the subsequent sticker wall task.
• The instructions should more clearly state what it expects the student to return
• The instructions seem to not use the formatting used elsewhere, here is an example of how other exercises show what needs to be done:

julia> knight_awake = true;

julia> can_do_fast_attack(knight_awake)
false