Fix incorrect curl command in API docs

The docs suggested that you only needed to use your api-base-url in
the curl command, but actually it needs to have /organizations appended
to it in order to work correctly. This makes it consistent with other
commands on this page which explicitly show a url to use.

I've also take the opportunity to reorder the curl arguments so the url
comes last, and use the long version of the option names for clarity.

Fixes https://github.com/mysociety/popit/issues/561