Update link to selectize.js (#2836)

Author: heyvaleria

app/views/fields/has_many/_form.html.erb

@@ -16,7 +16,7 @@ and is augmented with [Selectize].
   Contains helper methods for displaying a collection select box.
 
 [1]: http://www.rubydoc.info/gems/administrate/Administrate/Field/HasMany
-[Selectize]: http://brianreavis.github.io/selectize.js
+[Selectize]: https://github.com/selectize/selectize.js
 %>
 
 <div class="field-unit__label">