docs: add the unknown link in week1_day1_task1's docs (#39)

mini-lsm-book/src/week1-01-memtable.md

@@ -23,7 +23,7 @@ In this task, you will need to modify:
 src/mem_table.rs
 ```
 
-Firstly, let us implement the in-memory structure of an LSM storage engine -- the memtable. We choose [crossbeam's skiplist implementation](link) as the data structure of the memtable as it supports lock-free concurrent read and write. We will not cover in-depth how a skiplist works, and in a nutshell, it is an ordered key-value map that easily allows concurrent read and write.
+Firstly, let us implement the in-memory structure of an LSM storage engine -- the memtable. We choose [crossbeam's skiplist implementation](https://docs.rs/crossbeam-skiplist/latest/crossbeam_skiplist/) as the data structure of the memtable as it supports lock-free concurrent read and write. We will not cover in-depth how a skiplist works, and in a nutshell, it is an ordered key-value map that easily allows concurrent read and write.
 
 crossbeam-skiplist provides similar interfaces to the Rust std's `BTreeMap`: insert, get, and iter. The only difference is that the modification interfaces (i.e., `insert`) only require an immutable reference to the skiplist, instead of a mutable one. Therefore, in your implementation, you should not take any mutex when implementing the memtable structure.