bpo-38244: [Documentation] Fix unreliable link to sever in ftplib.rst

[Kriyszig]

• The link to ftp.debian.org is unreliable due to a failed redirect. Instead the main URL to ftp.us.debian.org is preferred

https://bugs.python.org/issue38244

[ghost]

Please remove 3 spaces before the comment, so that these # can be aligned.

[Kriyszig]

Addressed in 971d84f
Sorry I didn't notice that 😅 Thanks for pointing it out

[aeros]

Thanks for the PR @Kriyszig.

While I can understand the intent of this PR, I don't think we should recommend a server that's specific to a single country (if we can avoid doing so). This would not work as well for those on the opposite side of the world from the US. According to https://www.debian.org/mirror/list, it is recommended to use the following format to access their primary FTP mirrors:

 ftp.<country>.debian.org 

The only problem with using the above directly is that it wouldn't be valid in literal code, so that's up to the experts of ftplib to decide what should be used instead. But either way, we should stop recommending ftp.debian.org. From the debian-mirrors-anounce ML:

In the future, it may get services reduced, or shut down, or converted into
a globally load-balanced name, or whatever. Please don't use it.

If you're using it now, please switch to a country-based DNS name
such as ftp.us.debian.org, ftp.ca.debian.org, ftp.uk.debian.org, ...
The list of those servers is at http://www.debian.org/mirror/list

[aeros]

Expert for ftplib:
/cc @giampaolo

[giampaolo]

As long as ftp.us.debian.org is up, I'm OK with the PR as-is. If in the future we can find a better alternative we can just change it again.

[bedevere-bot]

@giampaolo: Please replace # with GH- in the commit message next time. Thanks!