Update documentation links in directory.dart

[gisborne]

Updated documentation links for file handling resources.

The page previously linked to no longer exists. This is a reasonable alternative.

[copybara-service]

Thank you for your contribution! This project uses Gerrit for code reviews. Your pull request has automatically been converted into a code review at:

https://dart-review.googlesource.com/c/sdk/+/471028

Please wait for a developer to review your code review at the above link; you can speed up the review if you sign into Gerrit and manually add a reviewer that has recently worked on the relevant code. See CONTRIBUTING.md to learn how to upload changes to Gerrit directly.

Additional commits pushed to this PR will update both the PR and the corresponding Gerrit CL. After the review is complete on the CL, your reviewer will merge the CL (automatically closing this PR).

[mraleph]

I think the link has to be retargeted to: https://dart.dev/libraries/dart-io#files-and-directories

[mraleph]

Seems stale