435 correct sftp chroot top dir to mnt3

[FroggyFlox]

Fixes #435 @phillxnet, ready for review.

This pull request's proposal

As detailed in #435, our docs incorrectly detail that an SFTP export is mounted at /mnt2 while it is actually mounted at /mnt3. This pull request corrects this.

Checklist

• [x] With the proposed changes no Sphinx errors or warnings are generated.
• [x] I have added my name to the AUTHORS file, if required (descending alphabetical order).

[phillxnet]

PR product PRODUCTION published.