Remove .css from default extensions in readme.md

.css has been removed from the default to comply with the Sass spec:
https://sass-lang.com/documentation/at-rules/import#importing-css
sass/node-sass#2362
https://github.com/sass/libsass/releases/tag/3.5.3

readme.md

@@ -71,7 +71,7 @@ Directories to use when resolved `@import` directives.
 #### extensions
 
 Type: `Array`
-Default: `['scss', 'css', 'sass']`
+Default: `['scss', 'sass']`
 
 File types to be parsed.