Update TestFlight link in README

[wi-ry]

Checklist

• [x] I have described what this PR contains

Choose one of the following two options:

• • [x] This PR does not introduce major changes
• • [ ] This PR introduces major changes, and I have consulted the Mlem Development Matrix room

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• • [x] This PR does not change the UI in any way
• • [ ] This PR adds new UI elements/changes the UI, and I have attached pictures or videos of the new / changed UI elements

Pull Request Information

About this Pull Request

Updated TestFlight link in README (Fixes #219)

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