Update usage.md: use 'file_url'

[mdsumner]

neither of 'bucket', 'path' are defined in this code

• minor doc fix

[TomNicholas]

Thanks @mdsumner . Looking at this more closely, there is a tab that displays different code for different backends. The current code with {bucket}/{path} is correct for the S3 case. Your fix with {file_url} would make it correct the Local case.

I think the correct thing to do would actually be to harmonize all the different options to all define a url variable correctly and just use that.

[mdsumner]

it did seem a bit inconsistent across the backends, I appreciate it's all changed quite quickly and settling around the obstore abstraction certainly seems good. I don't care about my actual PR just I needed an example that can run