Fix changelog script for consistent PR links

[lexnv]

We have noted during the release preparation (#481 ) that the script utilized for generating the changelog can result in inconsistent links.

Generated

- Return events from blocks skipped over during Finalization, too ([#473](git@github.com:paritytech/subxt/pull/473))

Expected

- Return events from blocks skipped over during Finalization, too ([#473](https://github.com/paritytech/subxt/pull/473))

The issue is related to how the pull request link is parsed.
For now, place a hardcoded link for uniform changelogs.

Testing Done

• ./scripts/generate_changelog.sh

[+] Latest release tag: v0.18.1
[+] Changelog

- Use RPC call to get account nonce ([#476](https://github.com/paritytech/subxt/pull/476))
- Generate release changelog based on commits ([#465](https://github.com/paritytech/subxt/pull/465))
- Update README ([#472](https://github.com/paritytech/subxt/pull/472))
- Return events from blocks skipped over during Finalization, too ([#473](https://github.com/paritytech/subxt/pull/473))
- Make EventSubscription and FilterEvents Send-able ([#471](https://github.com/paritytech/subxt/pull/471))

• shellcheck ./scripts/generate_changelog.sh

[TarikGul]

👍