Clarify the config api migration (#347)

The 0.7 example of the migration guide for the config api had a few issues. It passed the wrong datatype and it wasn't cleared what the argument of the closer was.

Co-authored-by: Charles <[email protected]>

content/learn/book/migration-guides/0.6-0.7/_index.md

@@ -213,7 +213,7 @@ fn camera_system(cameras: Query<&Camera, With<FirstPassCamera>>) {
 // 0.6
 struct Config(u32);
 
-fn local_config(local: Local<Config>) {
+fn local_is_42(local: Local<Config>) {
     assert_eq!(*local.0, 42);
 }
 
@@ -224,20 +224,15 @@ fn main() {
 }
 
 // 0.7
-struct Config(u32);
-
-fn local_config(local: u32) -> impl FnMut(ResMut<Config>) {
-    move |mut val| {
-        val.0 = local;
-
-        assert_eq!(val.0, 42);
+fn local_is_42(local: u32) -> impl FnMut() {
+    // This closure will be the system that will be executed
+    move || {
+        assert_eq!(local, 42);
     }
 }
 
 fn main() {
-        App::new()
-        .add_system(local_config(Config(42)))
-        .run();
+    App::new().add_system(local_is_42(42)).run();
 }
 ```