docs(input[checkbox]): update example to use ngModel best practices

It's not required for the example to function, but it prevents scope weirdness/unexpected
behavior when using directives (especially with ngTransclude!). I think it's a good pattern
to encourage and might prevent a bug down the road for for people who just scan for the
monospace font. See
[Understanding Scopes](https://github.com/angular/angular.js/wiki/Understanding-Scopes)
for mgModel best practices.

Closes #10851