fix: typo in onboarding flow for Node

[Lior539]

Context: https://posthog.slack.com/archives/C01V9AT7DK4/p1707506212271729

You call posthog.shutDownAsync() at the end, but in the project setup you tell us to call client.flush() instead.
posthog is not defined here in this context, and up at the top of the page you import PostHog which in JS would be different things. Leads me to believe this documentation should be updated

[posthog-bot]

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• chromium: 0 added, 0 modified, 0 deleted
• webkit: 0 added, 1 modified, 0 deleted (diff for shard 2)

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[github-actions]

Size Change: +134 B (0%)

Total Size: 2.22 MB

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Filename	Size	Change
frontend/dist/toolbar.js	2.22 MB	+134 B (0%)

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