Updating image_access.rst ordering instructions to match current API.

[wranglerr]

Summary

Updates the docs/imagery_access.rst to match the current API spec.

Background

I was attempting to follow the documentation to download some interesting imagery. The documentation for orders was using an old API.

I dug into the code to figure out what I believe the instructions should be. I confirmed they work by in a local jupyter notebook using gbdxtools.

[wranglerr]

@kostasthebarbarian Pinging you here since your name is at the top of the ordering.py and @drwelby since you're the only contributor in the recent history. This is a PR to update the documentation to reflect the current implementation.

Please let me know if there is someone else I should contact to get this merged in.

[drwelby]

@ldmalott Thanks for catching that!