Fix a typo in "schema.sql".

`jlineitems` needs to share a shard key with `jorders`, not with `orders`!
Surprisingly, this seems to be the only bug in my last three commits.

Throughput on a 2A/2L cluster --- measured in inserts-per-second (time-per-insert)
--- fluctuates around
     99K (10ms) for non-JSON data,
     22K (45ms) for JSON data with all six computed columns,
     40K (25ms) for JSON data extracting only lineitem.linenumber,
     75K (13ms) with both toggles in the "on" position.

So yes, extracting a bunch of computed columns from JSON is pretty slow.
JSON parsing itself roughly doubles the cost of an insert, and each extracted
column adds an extra 20% or so. It's not horrible, though, which is the main
thing.