fix word in readme (NMFR#66)

Thx for the help :)

Author: prime-time

README.md

@@ -8,7 +8,7 @@ It will search for CSS assets during the Webpack build and will optimize \ minim
 
 ### Solves [extract-text-webpack-plugin](http://github.com/webpack/extract-text-webpack-plugin) CSS duplication problem:
 
-Since [extract-text-webpack-plugin](http://github.com/webpack/extract-text-webpack-plugin) only bundles (merges) text chunks, if its used to bundle CSS, the bundle might have duplicate entries (chunks can be duplicate free but when merged, duplicate CSS can be created).
+Since [extract-text-webpack-plugin](http://github.com/webpack/extract-text-webpack-plugin) only bundles (merges) text chunks, if it's used to bundle CSS, the bundle might have duplicate entries (chunks can be duplicate free but when merged, duplicate CSS can be created).
 
 ## Installation: