docs: fix minor typo (#6223)

Author: tjcchen

docs/guide/css.md

@@ -1,6 +1,6 @@
 # Working with CSS
 
-Vue CLI projects comes with support for [PostCSS](http://postcss.org/), [CSS Modules](https://github.com/css-modules/css-modules) and pre-processors including [Sass](https://sass-lang.com/), [Less](http://lesscss.org/) and [Stylus](http://stylus-lang.com/).
+Vue CLI projects come with support for [PostCSS](http://postcss.org/), [CSS Modules](https://github.com/css-modules/css-modules) and pre-processors including [Sass](https://sass-lang.com/), [Less](http://lesscss.org/) and [Stylus](http://stylus-lang.com/).
 
 ## Referencing Assets