Errors found in the `control_flow` notebook

[despadam]

• [ ] Add more examples for one-line syntax.
• [ ] Provide multiple solutions: from a simpler one to a more advanced one.
• [ ] Add better explanation and more examples for continue and break statements.
• [ ] Add a section explaining the complexity that a nested loop adds to the code and how to avoid this.
• [ ] In exercise find a pair of numbers:
  • [ ] specify that we are looking for the first pair encountered,
  • [ ] specify that the two numbers should not be the same,
  • [ ] show how to solve this with both a nested and a single loop.