Editorial: Better align BestAvailableLocale with BCP 47

Per RFC 4647 section 3.4 - Lookup
(https://www.rfc-editor.org/rfc/rfc4647.html#section-3.4 ),
> Single letter or digit subtags (including both the letter 'x', which
> introduces private-use sequences, and the subtags that introduce
> extensions) are removed at the same time as their closest trailing
> subtag.

This is relevant for a requested locale such as "en-x-y-z-foo", which
should be truncated first to "en-x-y-z" and then to "en" ("y" is removed
at the same time as its closest trailing subtag "z" and "x" is removed
at the same time as its closest trailing subtag "y", which means all
three are removed in one step).

However, the change is still editorial because BestAvailableLocale is
never called with _availableLocales_ that would contain a language tag
like "en-x-y".

spec/negotiation.html

@@ -92,7 +92,8 @@ <h1>
         1. Repeat,
           1. If _availableLocales_ contains _candidate_, return _candidate_.
           1. Let _pos_ be the index into _candidate_ of the last occurrence of *"-"* (code unit 0x002D HYPHEN-MINUS). If that character does not occur, return *undefined*.
-          1. If _pos_ &ge; 2 and the substring of _candidate_ from _pos_ - 2 to _pos_ - 1 is *"-"*, set _pos_ to _pos_ - 2.
+          1. Repeat, while _pos_ &ge; 2 and the substring of _candidate_ from _pos_ - 2 to _pos_ - 1 is *"-"*,
+            1. Set _pos_ to _pos_ - 2.
           1. Set _candidate_ to the substring of _candidate_ from 0 to _pos_.
       </emu-alg>