Update spectroscopy-letters

Hi there!

I've done the changes as follows:
- .csl in the ID (line 7) but I don't see the extension in the ID codes of other styles...
- Documentation address is now http://www.tandfonline.com/action/authorSubmission?journalCode=lstl20&amp;page=instructions (line 9)
- Both ISSN updated
- and the changes proposed by zuphilip regarding the comma are also done. I tested it and now it looks very nice!

Thank you guys!