Fix link to gcsfs in tutorial

[dstansby]

The tutorial contains a link to gcsfs:

zarr-python/docs/tutorial.rst

Line 870 in ec4d216

Google Cloud Storage (`GCSMap

but this link is out of date and GCSMap no longer exists. It should be replaced with a link to https://gcsfs.readthedocs.io/en/latest/api.html#gcsfs.core.GCSFileSystem.get_mapper