update README

Author: serderovsh

README.rst

@@ -51,9 +51,12 @@ Example Smart Contract
     solidity_node = HttpProvider('https://api.trongrid.io')
     event_server = HttpProvider('https://api.trongrid.io')
 
-    tron = Tron(full_node,
-            solidity_node,
-            event_server)
+    tron = Tron(full_node=full_node,
+            solidity_node=solidity_node,
+            event_server=event_server)
+
+    # or default (tron = Tron())
+
 
     # Solidity source code
     contract_source_code = '''