fix: update README.md

[sophware]

Summary of Pull Request

Updates the README.md to demonstrate the use of 521 bits with ssh-keygen. Valid lengths are 256, 384, or 521.

Type of Pull Request

• This is a bugfix. type/bug
• This is an enhancement or feature. type/feature or type/enhancement
• This is a documentation update. type/docs
• This is a refactoring update. type/refactor
• This is a chore. type/chore
• This is something else.
  Please describe:

Related to Existing Issues

Closes #442

Test and Documentation Coverage

• Tests have been completed.
• Documentation has been added or updated.

Breaking Changes?

• Yes, there are breaking changes.
• No, there are no breaking changes.

[tenthirtyam]

Thank you for your contribution, @sophware.

Could we ask that you please update the pull request title and the commit details to follow the conventional commits structure as well as complete the pull request details and an accompanying linked docs issue per the CONTRIBUTING.md.

Here is an example for an update to the README.md: a4935c4

[tenthirtyam]

I've addressed creating and linking a documentation issue as per the contribution guidelines and have updated the pul request title.

[github-actions]

I'm going to lock this pull request because it has been closed for 30 days. This helps our maintainers find and focus on the active issues.
If you have found a problem that seems related to this change, please open a new issue and complete the issue template so we can capture all the details necessary to investigate further.