Fix issue id for `ComposeUnstableReceiver` in rules.md.

[ychescale9]

Update rules.md to change the related rule name to be the issue id ComposeUnstableReceiver instead of UnstableReceiverDetector.

[salesforce-cla[bot]]

Thanks for the contribution! Before we can merge this, we need @ychescale9 to sign the Salesforce Inc. Contributor License Agreement.

[ZacSweers]

Thanks!