Fix typo in worms example

Fix a typo in the `worms.rs` example, making all worms move in the same
direction instead of a random one.

Fixes #381

examples/worms.rs

@@ -1,8 +1,8 @@
 //! Worms
 //!
-//! Demonstrates simple use of particle trails.
+//! Demonstrates a simple use of particle trails.
 
-use std::f32::consts::{FRAC_PI_2, PI};
+use std::f32::consts::FRAC_PI_2;
 
 use bevy::{
  core_pipeline::{bloom::BloomSettings, tonemapping::Tonemapping},
@@ -58,7 +58,7 @@ fn setup(
  // scratch attribute.`
  let set_initial_angle_modifier = SetAttributeModifier::new(
  Attribute::F32_0,
- writer.lit(0.0).normal(writer.lit(0.0)).expr(),
+ writer.lit(0.0).normal(writer.lit(1.0)).expr(),
  );
 
  // Give each particle a random opaque color.