Update comments.cson

Minor fix: changed the 'scope' cson property to 'scopeName' to fix a warning message in the javascript console (toggle it with ctrl shift i):

> Failed to load grammar: /home/jo/.atom/packages/language-elm/grammars/comments.cson Error: Grammar missing required scopeName property: /home/jo/.atom/packages/language-elm/grammars/comments.cson
  at /usr/local/share/atom/resources/app/node_modules/first-mate/lib/grammar-registry.js:131:64
  at /usr/local/share/atom/resources/app/node_modules/season/lib/cson.js:244:61
  at fs.js:295:14
  at Object.oncomplete (fs.js:93:15)