fix: typo in md (iter->tier)

[cypppper]

I think the description in md:

mini-lsm/mini-lsm-book/src/week2-03-tiered.md

Line 79 in 2015ee1

The `num_iters` in the compaction simulator is set to 3. However, there are far more than 3 iters in the LSM state, which incurs large read amplification.

"num_iter" and "iter" should be "num_tier" and "tier"

as in the code:

mini-lsm/mini-lsm-starter/src/bin/mini-lsm-cli.rs

Line 330 in 2015ee1

num_tiers: 3,