Fix one-character typo in SystemParam docs (#19338)

# Objective

Remove errant "a" from docs.

(I'm assuming that this sort of trivial fix is easy enough to merge that
it's worth doing, but let me know if you'd prefer me to not bother.)

crates/bevy_ecs/src/system/system_param.rs

@@ -224,7 +224,7 @@ pub unsafe trait SystemParam: Sized {
     /// and creates a new instance of this param's [`State`](SystemParam::State).
     fn init_state(world: &mut World, system_meta: &mut SystemMeta) -> Self::State;
 
-    /// For the specified [`Archetype`], registers the components accessed by this [`SystemParam`] (if applicable).a
+    /// For the specified [`Archetype`], registers the components accessed by this [`SystemParam`] (if applicable).
     ///
     /// # Safety
     /// `archetype` must be from the [`World`] used to initialize `state` in [`SystemParam::init_state`].