docs: fix minor documentation bug

Author: xmatthias

docs/strategy-advanced.md

@@ -33,7 +33,7 @@ class AwesomeStrategy(IStrategy):
                 trade_entry_type = trade.get_custom_data(key='entry_type')
                 if trade_entry_type is None:
                     trade_entry_type = 'breakout' if 'entry_1' in trade.enter_tag else 'dip'
-                elif fills > 1:
+                elif len(fills) > 1:
                     trade_entry_type = 'buy_up'
                 trade.set_custom_data(key='entry_type', value=trade_entry_type)
         return super().bot_loop_start(**kwargs)