Update service_account_key.html.markdown

[SarahFrench]

Similar change as that in #1998

The docs resemble what a managed resource would say, instead of a data source

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[modular-magician]

Hi there, I'm the Modular magician. I've detected the following information about your changes:

Diff report

Your PR generated some diffs in downstreams - here they are.

google provider: Diff ( 1 file changed, 1 insertion(+), 1 deletion(-))
google-beta provider: Diff ( 1 file changed, 1 insertion(+), 1 deletion(-))

[SarahFrench]

@modular-magician reassign-reviewer

[github-actions]

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