docs: remove X from kind

[tenstad]

It's unclear how one should name XRD kinds.
Given the following:

docs/content/v2.0/get-started/get-started-with-composition.md

Lines 128 to 134 in 3b8bad4

	name: apps.example.crossplane.io
	spec:
	scope: Namespaced
	group: example.crossplane.io
	names:
	kind: App
	plural: apps

should the kinds within composite-resource-definitions.md also be rewritten to remove X?

If not, feel free to just close this PR.

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