Change 'dependencies' to 'devDependencies'. (#14736)

Because it says to run npm install with --save-dev flag
"dependencies": {
    "@wordpress/scripts": "3.1.0"
}
```
Should say:
```
"devDependencies": {
    "@wordpress/scripts": "3.1.0"
}
```

docs/designers-developers/developers/tutorials/javascript/js-build-setup.md

@@ -86,7 +86,7 @@ After installing, a `node_modules` directory is created with the modules and the
 Also, if you look at package.json file it will include a new section:
 
 ```json
-"dependencies": {
+"devDependencies": {
 	"@wordpress/scripts": "3.1.0"
 }
 ```