Add pull request number

Author: DougGregor

Release Notes/602.md

@@ -14,7 +14,7 @@
 
 - `DiagnosticsFormatter` has a new method, `formattedMessage`, that formats a diagnostic message without any corresponding syntax node.
   - Description: Some tools want to use the diagnostics formatter to produce diagnostics that don't relate to source code, or for which the source code isn't available. This API allows them to do so while maintaining consistent presentation.
-  - Pull Request: TBD
+  - Pull Request: https://github.com/swiftlang/swift-syntax/pull/3059
   - Migration steps: None required.
 
 - `FixIt.Change` has a new case `replaceText` that performs a textual replacement of a range of text with another string.