Add missing = (#2608)

Author: stzn

proposals/0434-global-actor-isolated-types-usability.md

@@ -166,7 +166,7 @@ class NonSendable {}
 func test() {
   let ns = NonSendable()
 
-  let closure { @MainActor in
+  let closure = { @MainActor in
     print(ns)
   }