Fix to video links for Exercises 1.2 and 1.3

Visitors to the online workshop who had tried to watch the introductory videos to Exercises 2 and 3 in Chapter 1 were disappointed to see the same introduction for those exercises as for Exercise 1 of Chapter 1. This was because of a coding error on the website.

The introductions to Exercises 2 and 3 of Chapter 1 can now be viewed in glorious colour.

• Exercise 2
  • discusses further hand calculations of detection probability using ducknest data
• Exercise 3
  • provides the first introduction to the use of Program Distance for analysis of ducknest data