Update chapter1.md

[Slyderek]

• L716-740: When run, the first two sqrt python functions give error as x_n is undefined, unlike the third sqrt code that uses generator. Therefore, I added and initialized x_n; x <- x_n after each of the approximation step.

• L826-831: Indentation reduced.

[TikhonJelvis]

Looks right. Thanks for the fixes!

[TikhonJelvis]

Next time I work on a book, I'm definitely going to set something up to run and test code samples from the manuscript :P