unicode_normalize/normalize.rb: rename variable

* lib/unicode_normalize/normalize.rb (hangul_decomp_one): use more
  descriptive name.  leave [SLVT]BASE and [LVTNS]COUNT as they are
  vague names.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@48055 b2dd03c8-39d4-4d8f-98ff-823fe69b080e