docs: fix typo (#169)

Author: stephan281094

README.md

@@ -13,7 +13,7 @@
 [![coverage][cover]][cover-url]
 [![chat][chat]][chat-url]
 
-This plugin extract CSS into separate files. It creates a CSS file per JS file which contains CSS. It supports On-Demand-Loading of CSS and SourceMaps.
+This plugin extracts CSS into separate files. It creates a CSS file per JS file which contains CSS. It supports On-Demand-Loading of CSS and SourceMaps.
 
 It builds on top of a new webpack v4 feature (module types) and requires webpack 4 to work.