fix typo: stdio gui with no wallet

same as https://github.com/spesmilo/electrum/pull/3223 - should have been included there

electrum/gui/stdio.py

@@ -26,7 +26,7 @@ class ElectrumGui(BaseElectrumGui):
         BaseElectrumGui.__init__(self, config=config, daemon=daemon, plugins=plugins)
         self.network = daemon.network
         storage = WalletStorage(config.get_wallet_path())
-        if not storage.file_exists:
+        if not storage.file_exists():
             print("Wallet not found. try 'electrum create'")
             exit()
         if storage.is_encrypted():