fix: added callout section for f-strings

Author: Scott Archer-Nicholls

episodes/01-introduction.md

@@ -189,6 +189,8 @@ else:
 
 :::::::::::::::::::::::::::::::::::::::::::::::
 
+:::::::::::::::::::::::::::::::::::::::::  callout
+
 ## Formatting Variables and Objects in Strings
 
 Throughout this course, we will be printing the values of variables inside strings to demonstrate what the code does. There are a number of ways to this in Python; in this course we will be using `f-strings` which are the recommended string interpolation syntax from version 3.6 onwards.