Fix usage instructions

This lib's rad, btw.

Author: danny-andrews

README.md

@@ -58,7 +58,7 @@ Like so:
   //...
   "plugins": [
     // ...
-    "mavrin/stylelint-declaration-use-css-custom-properties"
+    "@mavrin/stylelint-declaration-use-css-custom-properties"
   ],
   rules: {
     //...