fix pr template links

[kiftio]

What changes are you making?

Relative links don't work in the PR template, as they resolve relative to the current URL

E.g. clicking on the tip at the bottom in any current PR leads you to this URL, which is not intended https://github.com/Shopify/checkout-sheet-kit-swift/compare/CONTRIBUTING.md?expand=1

similarly, the changelog link resolves to https://github.com/Shopify/checkout-sheet-kit-swift/compare/CHANGELOG.md?expand=1

We can fix the links by just using absolute URLs

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Also some minor markdown link formatting changes

How to test

n/a

Before you merge

[!IMPORTANT]

• [ ] I've added tests to support my implementation
• [x] I have read and agree with the Contribution Guidelines.
• [x] I have read and agree with the Code of Conduct.
• [ ] I've updated the README.

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Checklist for releasing a new version

- [ ] I have bumped the version number in the [`podspec` file](https://github.com/Shopify/checkout-kit-swift/blob/main/ShopifyCheckoutKit.podspec#L2). - [ ] I have bumped the version number in the [README](https://github.com/Shopify/checkout-kit-swift/blob/main/README.md#packageswift). - [ ] I have added a [Changelog](./CHANGELOG.md) entry.

[!TIP] See the Contributing documentation for instructions on how to publish a new version of the library.