Merge ElementsProject#688: fix config err

eef2219 fix config err (Chengcheng Zhang)

Pull request description:

  I was confused to instructions in [Elements code tutorial](https://elementsproject.org/elements-code-tutorial/working-environment):

  > `cp ~/elements/contrib/assets_tutorial/elements2.conf ~/elementsdir2/elements.conf`

  But, in `~/elements/contrib/assets_tutorial/elements2.conf`, the network type is not specified at the beginning.

  I got an error after `e2-cli`:

  ```
  $ e2-cli getwalletinfo
  error: Could not connect to the server 127.0.0.1:7041

  Make sure the daemon server is running and that you are connecting to the correct RPC port.
  ```

  because elements default network is liquid, the port is 7041, so if you don't point the correct chain, you wouldn't get correct info.

  So I think the correct elements2.conf as below:

  ```
  chain=elementsregtest

  rpcuser=user2
  rpcpassword=password2
  elementsregtest.rpcport=18885
  elementsregtest.port=18887
  elementsregtest.connect=localhost:18886

  daemon=1
  listen=1
  txindex=1

  #fedpegscript=51<pubkey>51ae
  #signblockscript=51<pubkey2>51ae

  mainchainrpcport=18888
  mainchainrpcuser=user3
  mainchainrpcpassword=password3
  validatepegin=1

  initialfreecoins=2100000000000000
  ```

  I got correct result:

  ```
  $ e2-cli -getinfo
  {
    "version": 189900,
    "protocolversion": 70015,
    "walletversion": 169900,
    "balance": {
      "bitcoin": 21000000.00000000
    },
    "blocks": 0,
    "timeoffset": 0,
    "connections": 0,
    "proxy": "",
    "difficulty": null,
    "testnet": false,
    "keypoololdest": 1565173696,
    "keypoolsize": 1000,
    "paytxfee": 0.00000000,
    "relayfee": 0.00001000,
    "warnings": ""
  }
  ```

Tree-SHA512: c49303328fe5dde6682a1a8cde88350f35af081bb54d0d1dd5072a4c92cb043ed0b84303956a692bc4bc58f5741eae5664f8ffbe9ca795ca7f632c98d43586dd

contrib/assets_tutorial/elements2.conf

@@ -1,3 +1,5 @@
+chain=elementsregtest
+
 rpcuser=user2
 rpcpassword=password2
 elementsregtest.rpcport=18885