Updated readme.md to reflect new Sass update, which automatically output...

I was having issues after updating to the latest version of Sass and `gulp-ruby-sass`. I attempted to reference `readme.md` after receiving error messages about the deprecated `sourcemap` option, but it seemed outdated. After inspecting `index.js` it became evident to me that the code itself had been updated, but not the readme.
there is a `{` missing in that line.