Fix data storage key exmaple

[dstansby]

Near the top of the spec it says:

All chunk or other data of an array is stored under the key prefix determined by its path. For a root array, the key prefix is obtained from the metadata document key by stripping the trailing zarr.json. For example, for a root array, the prefix is the empty string. For a non-root array with hierarchy path /foo/bar, the prefix is foo/bar/.

As such, I don't think there should be a c level in the example I've edited?

[dstansby]

Ah maybe not, now I've read the rest of the spec! It's not super clear that in the default implementation that c is a special path.