docs: Fix incorrect cycle_modes function documentation (#18237)

Update the documentation comment to correctly state that the function
responds to spacebar presses rather than mouse clicks. This aligns the
documentation with the actual implementation, which was already using
keyboard input.

# Objective

- Fix incorrect documentation comment for the cycle_modes function that
inaccurately described using mouse input when the implementation
actually uses spacebar.
- This ensures documentation accuracy and prevents confusion for new
learners.

## Solution

- Updated the documentation comment to correctly state that the function
responds to spacebar presses rather than mouse clicks.
- The code functionality remains unchanged.

examples/window/low_power.rs

@@ -112,7 +112,7 @@ pub(crate) mod test_setup {
         window::RequestRedraw,
     };
 
-    /// Switch between update modes when the mouse is clicked.
+    /// Switch between update modes when the spacebar is pressed.
     pub(crate) fn cycle_modes(
         mut mode: ResMut<ExampleMode>,
         button_input: Res<ButtonInput<KeyCode>>,