Fix typo List<TripSummary> to List<BookingSummary> in ui-layer.md

[tarundhot]

Description of what this PR is changing or adding, and why: Fixed a simple typo of view model exposed data is a User Object, as well as the user's saved itineraries which are exposed as an object of type List

Presubmit checklist

• This PR is marked as draft with an explanation if not meant to land until a future stable release.
• This PR doesn’t contain automatically generated corrections (Grammarly or similar).
• This PR follows the Google Developer Documentation Style Guidelines — for example, it doesn’t use i.e. or e.g., and it avoids I and we (first person).
• This PR uses semantic line breaks of 80 characters or fewer.

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[sfshaza2]

lgtm

[sfshaza2]

/gcbrun

[sfshaza2]

@tarundhot, can you sign the CLA? Yes, even for a small PR like this. Thx!!

[flutter-website-bot]

Visit the preview URL for this PR (updated for commit 04e8f9a):

https://flutter-docs-prod--pr12127-patch-2-mqsovadb.web.app

[sfshaza2]

lgtm