Clarify that spelling_language strings must be exactly two or five characters

[cxw42]

Per #41,

The format [of a spelling_language value] is ss or ss-TT.

I intended ss to stand for exactly two characters and TT to stand for exactly two characters. However, as @gustaphe commented in https://github.com/editorconfig/editorconfig-vim/pull/210#issuecomment-2043070775 , TT could be read as a variable standing for any ISO 3166 code.

Update the specification to clarify that ss represents exactly two characters and TT represents exactly the two characters of an ISO 3166-1 alpha-2 code. Therefore, a spelling_lanugage string as a whole must be exactly two chars (ss) or exactly five chars (ss-TT).

[ffes]

It is always a good thing to clarify things if they turn out to be unclear!

Just a question. I guess the T in TT stands for territory, but what does the s in ss stand for?

[cxw42]

You know, I can't remember why I picked that! 😆 I would guess either (1)"s" for spelling, or (2)the letter before "t".