Fix Example JSON document for Array metadata.

[zoj613]

JSON array metadata document in the Array metadata section is ill-formed. the codecs field contains just the gzip codec configuration, even though the spec requires the codec chain to always have one array->bytes codec. That is in the example the gzip codec should be replaced with something more sensible like {"name": "bytes", "configuration": "big"} to be considered correct. This commit fixes that.

[d-v-b]

thanks for the fix! I will use this opportunity to suggest that the kind of mistake you fixed here would be less likely if we used a more structured JSON representation for the codecs, c.f. https://github.com/zarr-developers/zarr-specs/issues/298