Format ref page#246

[ramGranell]

If this pull request addresses an open issue on the repository, please add 'Closes #NN' below, where NN is the issue number.

Closes #246

Please briefly summarise the changes made in the pull request, and the reason(s) for making these changes.

I have modified the file learners/reference.md to harmonise the format as it was requested in the ticket. In addition I performed small modifications. The changes I performed can be summarised here:

1. After each command is defined in the references, there is always "-" symbol
2. All the verbs after the command/wildcard are in infinitive and not third-person
3. When flags are explained, I put always the command before the flag being consistent with the first "ls" example
4. Vertical bar character is not a flag of grep command and I added a separated command. I added also a small example using previous commands
5. I added a flag for the sort command as its normal behaviour is sorting using alphabetic characters

If any relevant discussions have taken place elsewhere, please provide links to these.

For more guidance on how to contribute changes to a Carpentries project, please review the Contributing Guide and Code of Conduct.

Please keep in mind that lesson Maintainers are volunteers and it may be some time before they can respond to your contribution. Although not all contributions can be incorporated into the lesson materials, we appreciate your time and effort to improve the curriculum. If you have any questions about the lesson maintenance process or would like to volunteer your time as a contribution reviewer, please contact The Carpentries Team at [email protected].

[github-actions]

🆗 Pre-flight checks passed 😃

This pull request has been checked and contains no modified workflow files, spoofing, or invalid commits.

It should be safe to Approve and Run the workflows that need maintainer approval.