fix autoprefixer link

Author: dbox

readme.md

@@ -1,7 +1,7 @@
 # Knapsack
 A handy bag of Sass mixins and utilities, and the Sass little brother to Stylus's [Axis](http://www.github.com/jenius/axis/) Library.
 
-Knapsack only provides pure css as defined by the w3c spec any browser prefixing  can and should be applied by [auto-prefixer] ([https://github.com/postcss/autoprefixer](https://github.com/postcss/autoprefixer)).
+Knapsack only provides pure css as defined by the w3c spec any browser prefixing  can and should be applied by [auto-prefixer](https://github.com/postcss/autoprefixer).
 
 > **note**: This library is in early development and new mixins are being added quickly, breaking changes will follow [semver in development](http://markup.im/#q4_cRZ1Qs) until 1.0.