Update client initialization in README (tikv#503)

 

Signed-off-by: Benjamin Kane <92759008+bkane-msft@users.noreply.github.com>

Author: bkane-msft

README.md

@@ -33,7 +33,7 @@ Raw mode:
 ```rust
 use tikv_client::RawClient;
 
-let client = RawClient::new(vec!["127.0.0.1:2379"], None).await?;
+let client = RawClient::new(vec!["127.0.0.1:2379"]).await?;
 client.put("key".to_owned(), "value".to_owned()).await?;
 let value = client.get("key".to_owned()).await?;
 ```
@@ -43,7 +43,7 @@ Transactional mode:
 ```rust
 use tikv_client::TransactionClient;
 
-let txn_client = TransactionClient::new(vec!["127.0.0.1:2379"], None).await?;
+let txn_client = TransactionClient::new(vec!["127.0.0.1:2379"]).await?;
 let mut txn = txn_client.begin_optimistic().await?;
 txn.put("key".to_owned(), "value".to_owned()).await?;
 let value = txn.get("key".to_owned()).await?;