docs: add missing space in docs

Author: xmatthias

docs/strategy-callbacks.md

@@ -634,7 +634,7 @@ class AwesomeStrategy(IStrategy):
 
 ## Custom order price rules
 
-By default, freqtrade use the orderbook to automatically set an order price([Relevant documentation](configuration.md#prices-used-for-orders)), you also have the option to create custom order prices based on your strategy.
+By default, freqtrade use the orderbook to automatically set an order price ([Relevant documentation](configuration.md#prices-used-for-orders)), you also have the option to create custom order prices based on your strategy.
 
 You can use this feature by creating a `custom_entry_price()` function in your strategy file to customize entry prices and `custom_exit_price()` for exits.