Update scoped-css.md

FF21 now supports this feature (woot!). The previous blurb mentioned a Chrome Canary reference (the "about:flags" bit), but it no longer seems to be valid. I didn't see any reference to scoped CSS, and it wasn't working in Canary.
Merge pull request #258 from benkeen/patch-2

Update Scoped CSS to reflect availability in Firefox Stable and remove note on Chrome