Update PostCSS transform example (fkling#148)

to match preferred code style in transform plugins

for fkling#147

see postcss/postcss-plugin-boilerplate@4d05aab

src/parsers/css/transformers/postcss/codeExample.txt

@@ -2,9 +2,9 @@ import * as postcss from 'postcss';
 
 export default postcss.plugin('postcss-reverse-props', (options = {}) => {
     // Work with options here
-    return css => {
+    return root => {
         // Transform CSS AST here
-        css.walkRules(rule => {
+        root.walkRules(rule => {
             // Transform each rule here
             rule.walkDecls(decl => {
                 // Transform each property declaration here