Update CHANGELOG.md (swiftlang#68619)

* Add the Swift 5.9 release date.
* Fix indentation of example code.

Author: benrimmington

CHANGELOG.md

@@ -9,14 +9,16 @@
 
   Member macros can specify a list of protocols via the `conformances` argument to the macro role. The macro implementation will be provided with those protocols that are listed but have not already been implemented by the type to which the member macro is attached, in the same manner as extension macros.
 
-   ```swift
-   @attached(member, conformances: Decodable, Encodable, names: named(init(from:), encode(to:)))
-@attached(extension, conformances: Decodable, Encodable, names: named(init(from:), encode(to:)))
-macro Codable() = #externalMacro(module: "MyMacros", type: "CodableMacro")
-   ```
+  ```swift
+  @attached(member, conformances: Decodable, Encodable, names: named(init(from:), encode(to:)))
+  @attached(extension, conformances: Decodable, Encodable, names: named(init(from:), encode(to:)))
+  macro Codable() = #externalMacro(module: "MyMacros", type: "CodableMacro")
+  ```
 
 ## Swift 5.9
 
+### 2023-09-18 (Xcode 15.0)
+
 * [SE-0382][], [SE-0389][], [SE-0394][], [SE-0397][]:
 
   Swift 5.9 includes a new macro system that can be used to eliminate boilerplate and provide new forms of expressive APIs. Macros are declared with the new `macro` introducer: