Reduce default sensitivity for free_cam controller (bevyengine#21451)

# Objective

As pointed out by @BigWingBeat in 
bevyengine#20215 (comment)
the default mouse sensitivity of the free_cam controller is crazy high.

## Solution

Divide the sensitivity value by 5. 10 was too much, but 5 feels about
right for a "careful and controlled scene editor".

## Testing

`cargo run --example 3d_gizmos --features="free_cam"`

Author: alice-i-cecile

crates/bevy_camera_controller/src/free_cam.rs

@@ -106,7 +106,7 @@ impl Default for FreeCam {
         Self {
             enabled: true,
             initialized: false,
-            sensitivity: 1.0,
+            sensitivity: 0.2,
             key_forward: KeyCode::KeyW,
             key_back: KeyCode::KeyS,
             key_left: KeyCode::KeyA,