Use absolute link for the newline wikipedia in book3/07-files.mkd.

[cadenzasong]

Line 567-570:

For most systems, there are applications to convert from one format to
another. You can find them (and read more about this issue) at
wikipedia.org/wiki/Newline. Or, of course,
you could write one yourself.

I think the link should be absolute, otherwise it's opening:
https://www.py4e.com/html3/wikipedia.org/wiki/Newline
and the page does not exist.