update summary for week 2

Signed-off-by: Alex Chi <iskyzh@gmail.com>

mini-lsm-book/src/SUMMARY.md

@@ -38,7 +38,7 @@
   - [Leveled Compaction Strategy](./week2-04-leveled.md)
   - [Manifest](./week2-05-manifest.md)
   - [Write-Ahead Log (WAL)](./week2-06-wal.md)
-  - [Snack Time: Batch Write](./week2-07-batch-write.md)
+  - [Snack Time: Batch Write and Checksums](./week2-07-snacks.md)
 
 - [Week 3: MVCC](./week3-overview.md)
 

mini-lsm-book/src/week2-07-batch-write.md

@@ -1 +0,0 @@
-# Snacks: Batch Write

mini-lsm-book/src/week2-07-snacks.md

@@ -0,0 +1,10 @@
+# Batch Write and Checksums
+
+<!-- ![Chapter Overview](./lsm-tutorial/week2-07-overview.svg) -->
+
+at the end of each week, we will have some easy, not important, while interesting things
+
+In this chapter, you will:
+
+* Implement the batch write interface.
+* Add checksums to the blocks, SST metadata, manifest, and WALs.