Fix typo in 600 release notes: isValidIdentifier -> isValidSwiftIdentifier

Author: ahoppen

Release Notes/600.md

@@ -32,7 +32,7 @@
   - 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: https://github.com/swiftlang/swift-syntax/pull/2379
 
-- `String.isValidIdentifier(for:)`
+- `String.isValidSwiftIdentifier(for:)`
   - Description: `SwiftParser` adds an extension on `String` to check if it can be used as an identifier in a given context.
   - Pull Request: https://github.com/swiftlang/swift-syntax/pull/2434