Fix output-decimal-marker

[krtko1]

[burnpanck]

Hi krtko1, thanks for this PR! While I haven't recently worked with this code, I recall that Literal allows you to use any LaTeX you wish as a output-decimal-marker, including for example the comma (,) - the full-stop (.) is simply the default. Therefore, the change you propose actually narrows the set of accepted literals to just those two instead of completely open. While these two are obviously the most common choices, I don't see a pressing need to restrict things too much, unless e.g. it's more close to the standard siunitx specification. Or is there in fact a problem with the Literal implementation that prevents it from accepting other options?

[krtko1]

when I tried do \SI[output-decimal-marker={,}]{1.1}{\metre} it shows 1.1 m. So Literal is probably not working.