Fix stake_currency return by Hyperopt

Hyperopt had BTC hard coded in the result. This commit  will display
the real stake_currency used.

If you used `"stake_currency": "USDT",` in your config file.
Before this commit you saw a message like:
"2 trades. Avg profit  0.13%. Total profit  0.00002651 BTC (0.0027Σ%). Avg duration 142.5 mins."

Now with the commit, we fix the wrong BTC currency:
"2 trades. Avg profit  0.13%. Total profit  0.00002651 USDT (0.0027Σ%). Avg duration 142.5 mins."

freqtrade/optimize/hyperopt.py

@@ -479,16 +479,16 @@ class Hyperopt(Backtesting):
             'result': result_explanation,
         }
 
-    @staticmethod
-    def format_results(results: DataFrame) -> str:
+    def format_results(self, results: DataFrame) -> str:
         """
         Return the format result in a string
         """
         return ('{:6d} trades. Avg profit {: 5.2f}%. '
-                'Total profit {: 11.8f} BTC ({:.4f}Σ%). Avg duration {:5.1f} mins.').format(
+                'Total profit {: 11.8f} {} ({:.4f}Σ%). Avg duration {:5.1f} mins.').format(
                     len(results.index),
                     results.profit_percent.mean() * 100.0,
                     results.profit_BTC.sum(),
+                    self.config['stake_currency'],
                     results.profit_percent.sum(),
                     results.duration.mean(),
                 )