[docs] Update startAsync() deprecated callout in Expo AuthSession f…

…or SDK 48 (expo#27406)

# Why

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Please describe the motivation for this PR, and link to relevant GitHub
issues, forums posts, or feature requests.
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Update deprecated callout for `startAsync()` to specify what hook to
migrate to (in this case `useAuthRequest()`).



# How

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How did you build this feature or fix this bug and why?
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Update SDK 48 docs since this is the last versioned doc that mentions
`startAsync()`.

# Test Plan

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reproduce your test, especially if this PR does not include automated
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Run docs locally and visit:
http://localhost:3002/versions/v48.0.0/sdk/auth-session/#authsessionstartasyncoptions

## Preview

![CleanShot 2024-03-04 at 01 12
35](https://github.com/expo/expo/assets/10234615/c814a310-5b80-4839-8b67-08f1eecca140)

# Checklist

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Please check the appropriate items below if they apply to your diff.
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- [x] Documentation is up to date to reflect these changes (eg:
https://docs.expo.dev and README.md).
- [ ] Conforms with the [Documentation Writing Style
Guide](https://github.com/expo/expo/blob/main/guides/Expo%20Documentation%20Writing%20Style%20Guide.md)
- [ ] This diff will work correctly for `npx expo prebuild` & EAS Build
(eg: updated a module plugin).