Add missing entry_tag argument to custom_entry_tag documentation

docs/strategy-callbacks.md

@@ -54,7 +54,7 @@ Called before entering a trade, makes it possible to manage your position size w
 class AwesomeStrategy(IStrategy):
     def custom_stake_amount(self, pair: str, current_time: datetime, current_rate: float,
                             proposed_stake: float, min_stake: float, max_stake: float,
-                            **kwargs) -> float:
+                            entry_tag: Optional[str], **kwargs) -> float:
 
         dataframe, _ = self.dp.get_analyzed_dataframe(pair=pair, timeframe=self.timeframe)
         current_candle = dataframe.iloc[-1].squeeze()