update mainnet

docs/go-nebulas/blockchain/mainnet.md

@@ -77,7 +77,7 @@ chain {
 Since Nebulas mainnet is running there for certain period of time, it will take quite some time to sync all the mainnet data from scratch. 
 
 For developers' convenience, we provided a offline data package, which already includes the data of more than 1 million blocks, you can download the package directly by following either link below (choose whichever is faster for you):
-- [`download from AWS s3`](https://s3-us-west-1.amazonaws.com/develop-center/data/mainnet/data.db.tar.gz)
+- [`download from AWS s3`](https://s3-us-west-1.amazonaws.com/develop-center/data/mainnet/data.db.tar)
 - [`download from Aliyun oss`](http://develop-center.oss-cn-zhangjiakou.aliyuncs.com/data/mainnet/data.db.tar.gz)
 
 > Please note that, the data package should be put under the same path of "datadir" as specified in your `config.conf` file.