Update README.md

Fix typo from `leve` to `level`

Author: reed-jones

README.md

@@ -1,6 +1,6 @@
 # postcss-selector-not [![CSS Standard Status](https://cssdb.org/badge/not-pseudo-class.svg)](https://cssdb.org/#not-pseudo-class) [![Build Status](https://travis-ci.org/postcss/postcss-selector-not.svg?branch=master)](https://travis-ci.org/postcss/postcss-selector-not)
 
-> PostCSS plugin to transform `:not()` W3C CSS leve 4 pseudo class to :not() CSS level 3 selectors
+> PostCSS plugin to transform `:not()` W3C CSS level 4 pseudo class to :not() CSS level 3 selectors
 
 http://dev.w3.org/csswg/selectors-4/#negation