update WriteBatch example (#686)

Signed-off-by: Nugine <nugine@foxmail.com>

README.md

@@ -83,11 +83,11 @@ fn main() {
     // NB: db is automatically freed at end of lifetime
     let mut db = DB::open_default("/path/for/rocksdb/storage").unwrap();
     {
-        let mut batch = WriteBatch::new(); // WriteBatch and db both have trait Writable
+        let batch = WriteBatch::new(); // WriteBatch and db both have trait Writable
         batch.put(b"my key", b"my value");
         batch.put(b"key2", b"value2");
         batch.put(b"key3", b"value3");
-        db.write(batch); // Atomically commits the batch
+        db.write(&batch).unwrap(); // Atomically commits the batch
     }
 }
 ```