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Fix help text for runnable plugin command

Fix help text for runnable plugin command: a task in repo2rlenv-commit-runtime-v2 (Harbor dataset). Title: Plugin command help text shows kubectl-plugin instead of kubectl plugin in usage line

The task

**Title:** Plugin command help text shows `kubectl-plugin` instead of `kubectl plugin` in usage line

Part of FineEnvs/repo2rlenv-commit-runtime.