Don't use old syntax in docs

api/cpp/docs/generated_code.md

@@ -115,7 +115,7 @@ similar to API that's created for your `.slint` component.
 For example the following `.slint` markup defines a global `Logic` singleton that's also exported:
 
 ```slint,ignore
-export global Logic := {
+export global Logic {
     callback to_uppercase(string) -> string;
 }
 ```

api/cpp/docs/types.md

@@ -31,7 +31,7 @@ code.
 For example, this `struct` in a `.slint` file
 
 ```slint,ignore
-export struct MyStruct := {
+export struct MyStruct {
     foo: int,
     bar: string,
 }

api/cpp/include/slint_interpreter.h

@@ -729,8 +729,8 @@ public:
     ///
     /// Example: imagine the .slint file contains the given global:
     /// ```slint,no-preview
-    ///    export global Logic := {
-    ///         callback to_uppercase(string) -> string;
+    ///    export global Logic {
+    ///         pure callback to_uppercase(string) -> string;
     ///    }
     /// ```
     /// Then you can set the callback handler