Demo app: update ios AppAuth dependency

[djm158]

Description

Fixes the following issue when running pod install from examples/demo/ios:

[!] CocoaPods could not find compatible versions for pod "AppAuth":
  In snapshot (Podfile.lock):
    AppAuth (= 1.7.5, >= 1.7.3)

  In Podfile:
    react-native-app-auth (from `../../../node_modules/react-native-app-auth`) was resolved to 8.0.1, which depends on
      AppAuth (>= 1.7.6)


You have either:
 * out-of-date source repos which you can update with `pod repo update` or with `pod install --repo-update`.
 * changed the constraints of dependency `AppAuth` inside your development pod `react-native-app-auth`.
   You should run `pod update AppAuth` to apply changes you've made.

Ran pod update AppAuth and committed the changes.

Additionally, updates the README with the correct path to the demo.

Steps to verify

Follow setup instructions in the README

Clone repository, checkout chore/update-AppAuth-demo-app, then run the following:

cd react-native-app-auth/examples/demo
yarn
(cd ios && pod install)
cd ..
npx react-native run-ios

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Latest commit: c1fe823

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Name	Status	Preview	Comments	Updated (UTC)
react-native-app-auth	✅ Ready (Inspect)	Visit Preview	💬 Add feedback	Jan 7, 2025 6:59pm

[JDMathew]

Verified update ✅