Update docs to make clear need for /api in endpoint requests

[maxachis]

I recently ran into an issue where I attempted to follow the documentation's advice and make endpoint requests using https://data-sources.pdap.io/.

However, I soon learned that attempting to do it like that from a simple programmatic request doesn't work. Instead, all endpoints need to be preceded by https://data-sources.pdap.io/api. This isn't mentioned in the documentation.

The documentation should be amended to include that.

[josh-chamberlain]

fixed with a gitbook PR: https://app.gitbook.com/o/-MXypK5ySzExtEzQU6se/s/-MXyolqTg_voOhFyAcr-/~/changes/495/api/endpoints/admin/~/overview