Add pull request number to release note

Author: DougGregor

Release Notes/511.md

@@ -30,7 +30,7 @@
 
 - `ThrowsClause`
   - Description: The `throwsSpecifier` for the effects nodes (`AccessorEffectSpecifiers`, `FunctionEffectSpecifiers`, `TypeEffectSpecifiers`, `EffectSpecifiers`) has been replaced with `throwsClause`, which captures both the throws specifier and the (optional) thrown error type, as introduced by SE-0413.
-  - Pull Request: TBD
+  - Pull Request: https://github.com/apple/swift-syntax/pull/2379
 
 ## API Behavior Changes