docs: correct according change config

Author: Scrum

readme.md

@@ -37,14 +37,11 @@ $ posthtml --help
 ```
 >**Note:** This package does not respect the order of patterns. First, all the negative patterns are applied, and only then the positive patterns.
 
-Also note that in case of using external config file, the config itself needs to be contained inside "posthtml" key:
 ```js
 module.exports = {
-  posthtml: {
-    plugins: {
-      'posthtml-plugin': {
-        foo: 'bar'
-      }
+  plugins: {
+    'posthtml-plugin': {
+      foo: 'bar'
     }
   }
 };