Update documentation for vue-multiselect 'trackBy'

[navjeetfgmnt]

As it states in vue-multiselect documentation it is a breaking change
The :key props has changed to :track-by, due to conflicts with Vue 2.0.
It causes select-options like 'hideSelected' and 'selectedLabel' to not work properly if using 'key' instead of 'trackBy'.
In the vueMultiSelect tab in documentation it still shows 'key' as a property https://icebob.gitbooks.io/vueformgenerator/content/fields/vuemultiselect.html