[fix]: Fix typo in w1d6

[Gleiphir2769]

"two" things -> "three" things

mini-lsm/mini-lsm-book/src/week1-06-write-path.md

Lines 29 to 33 in bcaab6f

	You will need to modify `LSMStorageInner::force_flush_next_imm_memtable` and `MemTable::flush`. In `LSMStorageInner::open`, you will need to create the LSM database directory if it does not exist. To flush a memtable to the disk, we will need to do two things:
	* Select a memtable to flush.
	* Create an SST file corresponding to a memtable.
	* Remove the memtable from the immutable memtable list and add the SST file to L0 SSTs.