mention positionals in Nothing example
It turns out the cause for the problem seen in #4271 is actually a special case handling where if there is only one input_def that isn't defined in positional we resolve to it https://dagster.phacility.com/source/dagster/browse/master/python_modules/dagster/dagster/core/definitions/i_solid_definition.py$74-79
I can't think of a good way to handle it more cleanly, so just use kwargs in the tutorial example and add some text to call it out.
Test Plan: eyes
Reviewers: owen, yuhan
Reviewed By: owen
Differential Revision: https://dagster.phacility.com/D8876