fix pandas (#249)

Author: HumphreyYang

lectures/pandas.md

@@ -695,6 +695,10 @@ def read_data(ticker_list,
     for tick in ticker_list:
         stock = yf.Ticker(tick)
         prices = stock.history(start=start, end=end)
+
+        # Change the index to date
+        prices.index = pd.to_datetime(prices.index.date)
+        
         closing_prices = prices['Close']
         ticker[tick] = closing_prices