Update implementation link to point to the PR that promotes exit tests to API

Author: grynspan

proposals/testing/NNNN-exit-tests.md

@@ -4,7 +4,7 @@
 * Authors: [Jonathan Grynspan](https://github.com/grynspan)
 * Status: **Awaiting review**
 * Bug: [apple/swift-testing#157](https://github.com/apple/swift-testing/issues/157)
-* Implementation: [apple/swift-testing#307](https://github.com/apple/swift-testing/pull/307)
+* Implementation: [apple/swift-testing#324](https://github.com/swiftlang/swift-testing/pull/324)
 * Review: ([pitch](https://forums.swift.org/t/pitch-exit-tests/78071))
 
 ## Introduction