Change 'save-as' short option to '-sa'

[migue802]

PR Checklist

• A description of the changes is added to the description of this PR.
• If there is a related issue, make sure it is linked to this PR.
• If you've fixed a bug or added code that should be tested, add tests!
• Documentation is updated

Description of changes

Both "--start" and "--save-as" had the short option name "-s", which lead to unexpected behaviour (running only as --save-as). This can be solved by changing one of them. I determined the one that fits more for change is the "save-as" option to "-sa".

Just to let you know, I'm new into this, I don't know if there should be a test for this, or documentation anywhere else, or if it even works (which I believe so, as it's the only reference to the arguments names, but I haven't tested it).

Thanks.