revert schedule change for debug render (#497)

revert schedule change for PhysicsDebugPlugin because it causes large lag

# Objective
fixes issue #496
recent scheduling changes from commit 4d082a7 causes PhysicsDebugPlugin too generate lots of lag when enabled

## Solution
change default schedule from FixedPostUpdate too PostUpdate

src/debug_render/mod.rs

@@ -89,7 +89,7 @@ impl PhysicsDebugPlugin {
 
 impl Default for PhysicsDebugPlugin {
     fn default() -> Self {
-        Self::new(FixedPostUpdate)
+        Self::new(PostUpdate)
     }
 }