Fix GKEStartPodOperator docstring default for on_finish_action

[Ayush-srivastava504]

Fixes: #58272

The docstring for GKEStartPodOperator stated the default value for on_finish_action as keep_pod.
Updated it to delete_pod to match the current behavior.

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[boring-cyborg]

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[shahar1]

Great job!

[shahar1]

Please refrain from making additional changes to this PR so I could see that the CI passes succesfully and merge. Thank you! :)

[Ayush-srivastava504]

Sure sir, I won’t make any more changes. Thank you!

[shahar1]

Small linting fix, I will apply the fix.

[Ayush-srivastava504]

Thanks sir ,
appreciate it a lot!

[boring-cyborg]

Awesome work, congrats on your first merged pull request! You are invited to check our Issue Tracker for additional contributions.