Update README.md (#909)

Fixes wrong variable name in usage demo

diff --git a/README.md b/README.md
index d69d18d..447b3ad 100644 (file)
--- a/README.md
+++ b/README.md
@@ -66,7 +66,7 @@ npm install clean-css
 var CleanCSS = require('clean-css');
 var input = 'a{font-weight:bold;}';
 var options = { /* options */ };
-var output = new CleanCSS(options).minify(source);
+var output = new CleanCSS(options).minify(input);
 ```
 
 ## Important: 4.0 breaking changes