Updated Stream Instructions for sip parameter in README (#615)

* Updated Stream Instructions to not use unneeded URL

* Updated Sip instructions to be more clear.

* Added back in the URL and additional comment for the data feed type of second example

Author: hinchcliffz

README.md

@@ -250,7 +250,7 @@ async def quote_callback(q):
 stream = Stream(<ALPACA_API_KEY>,
                 <ALPACA_SECRET_KEY>,
                 base_url=URL('https://paper-api.alpaca.markets'),
-                data_feed='iex')  # <- replace to SIP if you have PRO subscription
+                data_feed='iex')  # <- replace to 'sip' if you have PRO subscription
 
 # subscribing to event
 stream.subscribe_trades(trade_callback, 'AAPL')
@@ -270,7 +270,7 @@ ie:
 stream = Stream(<ALPACA_API_KEY>,
                 <ALPACA_SECRET_KEY>,
                 base_url=URL('https://paper-api.alpaca.markets'),
-                data_feed='iex',
+                data_feed='iex', # <- replace to 'sip' if you have PRO subscription
                 websocket_params =  {'ping_interval': 5}, #here we set ping_interval to 5 seconds 
                 )
 ```