Update go_router type safe route docs to reflect that the generated mixins are now public

[LiamKearn]

Title says all.

REF: #9626

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[gemini-code-assist]

Code Review

This pull request updates the documentation in packages/go_router/doc/type-safe-routes.md to use $HomeScreenRoute and $SongRoute instead of _$HomeScreenRoute and _$SongRoute. No review comments were provided, and I have no feedback to provide.

[stuartmorgan-g]

@chunhtai I would recommend requiring that this adopt snippets as part of the fix, rather than just changing the text without fixing the underlying problem that there's no verification that the code is accurate.