update cmip6 tutorial

[cisaacstern]

As noted in #105 (comment), the Postscript of the cmip6 tutorial should now be updated to reflect that dataset = 'IPSL-CM6A-LR.abrupt-4xCO2.r1i1p1f1.Lmon.cLeaf.gr' does work with the latest release of h5netcdf.

In addition, populating the keys argument of ConcatDim should be updated to reflect that:

The CSV file actually has a column labeled temporal subset which could be used directly to get your keys argument for ConcatDim.

[cisaacstern]

Per #133 (comment), this update also requires in Step 5, changing

meta_store = FSSpecTarget(fs_local, meta_dir.name)

to:

meta_store = MetadataTarget(fs_local, meta_dir.name)