[dagit] Clarify termination dialog language
Summary: Kicking off terminations is more of a request for termination, since we don't sit around waiting for the termination to complete. Clarify this in the termination dialog.
Test Plan: View TerminationDialog storybook, verify correct rendering.
Reviewers: dgibson, johann, prha
Reviewed By: dgibson
Differential Revision: https://dagster.phacility.com/D5409