docs($http): fix adding default header to get request example
docs($http): fix adding default header to get request example

Initially, `$httpProvider.defaults.headers.get` is `undefined`, so
`$httpProvider.defaults.headers.get['My-Header']='value'` will throw an
error.

Closes #4101

ammula88/angular.js,ammula88/angular.js,ammula88/angular.js,ammula88/angular.js,ammula88/angular.js