Fix motion_blur example instructions (#15852)

# Objective

Fixes a mistake in the migration done in #15591.

## Solution

Restore a line of instructions that was accidentally dropped.

## Testing

`cargo run --example motion_blur`

Tested that instructions make sense and text updates correctly when keys
are pressed.

examples/3d/motion_blur.rs

@@ -246,7 +246,7 @@ fn setup_ui(mut commands: Commands) {
  p.spawn(TextSpan::default());
  p.spawn(TextSpan::new("1/2: -/+ shutter angle (blur amount)\n"));
  p.spawn(TextSpan::new("3/4: -/+ sample count (blur quality)\n"));
- p.spawn(TextSpan::new("3/4: -/+ sample count (blur quality)\n"));
+ p.spawn(TextSpan::new("Spacebar: cycle camera\n"));
  });
 }