ES5 setup file correction

ES5 configuration proposed originally basically doesn't work. We're requiring 'enzyme' as Enzyme, but using configure() method out of nowhere:
```js
var Enzyme = require('enzyme');
var Adapter = require('enzyme-adapter-react-16');

configure({ adapter: new Adapter() });
```

I derived my variant from the original ES6 versino and it works fine for me.
Need it specifically for the .babelrc ["env", { "modules": false }] case.

docs/installation/react-16.md

@@ -41,10 +41,10 @@ ES5:
 <!-- eslint no-var: 0 -->
 ```js
 // setup file
-var Enzyme = require('enzyme');
+var enzyme = require('enzyme');
 var Adapter = require('enzyme-adapter-react-16');
 
-configure({ adapter: new Adapter() });
+enzyme.configure(Adapter);
 ```
 
 <!-- eslint no-var: 0 -->