Update PageRule docs regarding zone name

[clstokes]

As seen in #23, it's unclear that zone for PageRule is actually the zone name (e.g. example.com) and not the zone id.

We should update the docs (or alternatively the property name) to be more clear.

[lukehoban]

This package is automatically derived from https://github.com/terraform-providers/terraform-provider-cloudflare, so I don't think it would be a good idea to change the property names in our layer - possibly worth suggesting that in the upstream provider - though would be a breaking change there (and here).

[stack72]

I personally think the documentation needs to be better here - I will submit a fix upstream that makes it more clear that it's the zone name not the ID

[stack72]

Hi folks, I am going to close this out. It seems there is a change coming down the pipeline that deprecates the use of zone in favour of zone_id

https://github.com/terraform-providers/terraform-provider-cloudflare/pull/421

This will hit soon I believe and will be able to be more explicit about which to use