Add UPDATE to SQL lessons

[ethanwhite]

When showing students how to add and remove records it will be natural for them to ask how to modify an existing record. This demonstrates how to do this using Update in a few lines.

To avoid the relevant lesson becoming too much of a wall of text, this also splits the lesson into two sections.

[ethanwhite]

Looks like I'd forgotten to add the updated markdown version of the notebook, which I've done now.