docs(typo): from "mongoServer" to "mongod" (#53)

Sometimes you use the name "mongoServer" and sometimes "mongod" to represent the instance of MongodbMemoryServer. How could we improve the consistency of documentation?

Author: leonardomerlin

README.md

@@ -33,7 +33,7 @@ const mongod = new MongodbMemoryServer();
 const uri = await mongod.getConnectionString();
 const port = await mongod.getPort();
 const dbPath = await mongod.getDbPath();
-const dbName = await mongoServer.getDbName();
+const dbName = await mongod.getDbName();
 
 // some code