Missing dependencies in code example for 3rd party dependencies

Under Addon Authors guide, for Third Party Dependencies,
We need to include Funnel and mergeTrees dependencies

## CURRENT
```js
var map = require('broccoli-stew').map;

treeForVendor(defaultTree) {
var browserVendorLib = new Funnel();

browserVendorLib = map(browserVendorLib, (content) => `if (typeof FastBoot === 'undefined') { ${content} }`);

return new mergeTrees([defaultTree, browserVendorLib]);
}

included() {
// this file will be loaded in FastBoot but will not be eval'd
app.import('vendor/.js');
}
```
## EXPECTED
```js
var map = require('broccoli-stew').map;
var Funnel = require('broccoli-funnel');
var mergeTrees = require('broccoli-merge-trees');

treeForVendor(defaultTree) {
var browserVendorLib = new Funnel();

browserVendorLib = map(browserVendorLib, (content) => `if (typeof FastBoot === 'undefined') { ${content} }`);

return new mergeTrees([defaultTree, browserVendorLib]);
}

included() {
// this file will be loaded in FastBoot but will not be eval'd
app.import('vendor/.js');
}
```
Example code referred from
https://github.com/soulim/ember-cli-bootstrap-datepicker/pull/100/files#diff-168726dbe96b3ce427e7fedce31bb0bcR7