Fix doc in App::add_sub_app (bevyengine#7139)

# Objective

- Fix the name of function parameter name in docs

## Solution

- Change `f` to `sub_app_runner`

---

It confused me a bit when I was reading the docs in the autocomplete hint.
Hesitated about filing a PR since it's just a one single word change in the comment.
Is this the right process to change these docs?

crates/bevy_app/src/app.rs

@@ -1018,7 +1018,7 @@ impl App {
 
     /// Adds an [`App`] as a child of the current one.
     ///
-    /// The provided function `f` is called by the [`update`](Self::update) method. The [`World`]
+    /// The provided function `sub_app_runner` is called by the [`update`](Self::update) method. The [`World`]
     /// parameter represents the main app world, while the [`App`] parameter is just a mutable
     /// reference to the `SubApp` itself.
     pub fn add_sub_app(