Add some time for exercises into Lesson 5 - Repeating Actions with Loops

[Robin-Wardle]

Lesson 4 - Repeating actions with Loops - currently allocates "0 minutes" for the class exercises. Having taught this course, and been short of time due to the way that the lesson timings were used to plan the day, I suggest that 15 minutes be allowed in total for students to do the exercises.

There are 5 exercises: exercises 1-4 are simple but the first one introduces the range() facility which requires a bit of explanation. The last exercise introduces the enumerate() facility, which also requires explanation.