docs: fix a test example link

Fix Typescript Reference

Author: nodkz

README.md

@@ -143,7 +143,7 @@ const replSet = new MongoMemoryReplSet({
 
 ### Simple test with MongoClient
 
-Take a look at this [test file](https://github.com/nodkz/mongodb-memory-server/blob/master/src/__tests__/singleDB-test.js).
+Take a look at this [test file](https://github.com/nodkz/mongodb-memory-server/blob/master/src/__tests__/singleDB-test.ts).
 
 ### Provide connection string to mongoose