docs: fix graphql query to return failed runs

Summary:
https://dagster.slack.com/archives/C01U954MEER/p1624930409194700

These queries should be under test. Fixing the immediate problem first.

Test Plan: run query locally

Reviewers: sashank, sidkmenon

Reviewed By: sidkmenon

Differential Revision: https://dagster.phacility.com/D8586