fix: adjust real time data example code (#590)

Author: hiohiohio

docs/market_data.rst

@@ -158,7 +158,7 @@ methods to receive real-time data.
 
 .. code-block:: python
 
-    from alpaca.data import CryptoDataStream, StockDataStream
+    from alpaca.data.live import CryptoDataStream, OptionDataStream, StockDataStream
 
     # keys are required for live data
     crypto_stream = CryptoDataStream("api-key", "secret-key")