Changed table separator from single-hyphen (-) to triple-hyphen (---)

[arctus-io]

Changed table separator from single-hyphen (-) to triple-hyphen (---) and updated test. This change brings the resulting tables into compliance with the YouTrack Markdown implementation.

Fixes #29

[catcombo]

Hi @arctus-io !

Thank you very much for investigation and preparing the PR. YouTrack still accepts single-hyphen as a table separator, but I'm not against the changes to be fully compatible with the YouTrack documentation. Some tests are failing, but it shouldn't be a big problem to fix them. Could you also change commits messages to align with the https://www.conventionalcommits.org/en/v1.0.0/ and the commit history https://github.com/catcombo/jira2markdown/commits/master/

• for the first commit you can use commit message feat(tables): change table header delimiter to triple hyphen
• for the second one docs(readme): update table header conversion docs

[arctus-io]

@catcombo Thanks, for being open to changes.

I updated the remaining tests that failed and I (attempted to) fix the commit messages. Tests are passing on my side:

[catcombo]

@arctus-io Looks good to me. I removed the merge commit so now we have a clean commit history. I'll merge the PR now and publish a new package version. Thank you very much!