Docs: Update location references for the API endpoint

[KyleBlankRollins]

The current wording for where to find the Identity API endpoint in the Netlify UI can be a bit confusing. For example, the readme currently has:

APIUrl: The absolute path of the GoTrue endpoint. To find the APIUrl, go to Identity page of your Netlify site dashboard.

and

Also, make sure the Registration preferences under Identity settings in your Netlify dashboard are set to Open.

These references are easily conflated. I suggest updating them to:

APIUrl: The absolute path of the GoTrue endpoint. To find the APIUrl in the Netlify UI, go to your site and navigate to the Identity page.

and

Also, make sure Site settings > Identity settings > Registration preferences is set to Open.