Add an extra parameter 'show_in_rest' in custom taxonomy code snippet of developer doc.

[akhterarif]

An extra parameter show_in_rest has to be added in the following section.

​https://developer.wordpress.org/plugins/taxonomies/working-with-custom-taxonomies/#step-2-creating-a-new-plugin

It is not working with the current settings while creating post with Gutenburg editor the taxonomy section is not showing. If we add the show_in_rest to true in $args array of the code snippet it shows the taxonomy in post creation window.