fix postcss link

Author: chernobelenkiy

README.md

@@ -1,6 +1,6 @@
 # postcss-px-to-viewport [![NPM version](https://badge.fury.io/js/postcss-px-to-viewport.svg)](http://badge.fury.io/js/postcss-px-to-viewport)
 
-A plugin for [PostCSS](https://github.com/ai/postcss) that generates viewport units (vw, vh, vmin, vmax) from pixel units.
+A plugin for [PostCSS](https://github.com/postcss/postcss) that generates viewport units (vw, vh, vmin, vmax) from pixel units.
 
 Feel free to start watching and ⭐ project in order not miss the release or updates.