Update _vars.scss

Change comment on line-5 from '_vars.scss' to '_defaults.scss'. Current comment states "Any variables you find set in inuit.css’ `_vars.scss` that you ..." The core library directory inuit.css contains file named '_defaults.scss', not file '_vars.scss'. In fact the contents of '_vars.scss' (which we are presently reading) is intended to override '_default.scss'. This was confusing to this noobie -thanks, frannyshaw