Fix docs mistake in bevy_ecs::world

[berylllium]

Objective

• Correct a mistake in the rustdoc for bevy_ecs::world::World.

Solution

• The rustdoc wrongly stated that "Each component can have up to one instance of each component type.". This sentence should presumably be "Each Entity can have up to one instance of each component type.". Applying this change makes the prior sentence "Each [Entity] has a set of components." redundant.

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[LikeLakers2]

Perhaps something like

Each Entity has a set of components, but only one instance of each type of component.

so that it's clear that multiple types of components can exist, but only one of each type.

Suggested change

	/// Each [`Entity`] can have up to one instance of each type of component. Entity
	/// components can be created, updated, removed, and queried using a given [`World`].
	/// Each [`Entity`] has a set of components, but only one instance of each type of
	/// component. Entity components can be created, updated, removed, and queried using
	/// a given [`World`].