[go_router] Update redirect documentation example code for clarity

[elitree]

The redirection documentation page gives an example where the top level redirect may be used to send non-signed-in users to the signin page. The example code seems to be doing so for signed in users, however.

This is a documentation-only change.

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• [X] I updated pubspec.yaml with an appropriate new version according to the pub versioning philosophy, or this PR is exempt from version changes.
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